Forest - Are you in the 45-49 masters category and if so are you planning to take the world record from Colby Pearce, your fellow Boulderite? 
I live below sea level in the Netherlands (about -6 meters), so what to do with that? Do I get a malus?
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This author is banned from sport but I think he would have some suggestions if he would be open to assisting. https://www.53x12.com/training-at-altitude
When Iāve raced and trained around the US, Colombia and Rwanda, anywhere from Sea level to 14,130 ft, Iāve always enjoyed looking at the elevation correction in WKO4+. I would be interesting to ask Tim Cusick about their process to make that chart. Talking with Dr. Andrew Coggan and Allen Hunter might help as well, but I doubt Zwift sees the utility to invest in their knowledge.
I might be weird here but I always have felt best around 7,000-8,000 ft. There is a lightness of the air that just feels optimal. Above that my power/recovery drops off too much to really enjoy it. I know my power is higher in Ohio, than it was at altitude but the air is so heavy that is isnāt as nice to breath. Ah! I miss living/riding in CO. Going from the Evergreen Res. up to Juniper Pass ~ 11,000 ft was and is a ride that I reflect on fondly.
Johnathan - I donāt disagree that moving from elevation to another have advantages and disadvantages.
I used to live at 4500ā elevation and I was training and racing a lot. Then I traveled to the USA at sea level I did a bunch of training rides and set some good times. Now I live in the USA and I just dont seem to be able to match the times I set on that route, and those times were set on a 1976 6 speed with down tube shifters. I just think my body got used to the elevation and I am back to my performance prior to moving to the USA. But this is a sample size of one, so it does not hold a lot of credit. O and iām olderā¦ LOL
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Great data points here, Eric. Thanks for sharing all of that. It lines up with my exact experience living in Boulder, watching my power over long periods of time at this altitude, seeing how it drops when I go higher in the mountains for long periods (and stays that way after acclimatizing), and watching the effects at sea level, where I gain around 10% or 12% more powerā¦and it stays that way after Iāve been there for weeks.
Also, both Todayās Plan and Training Peaks now have āaltitude adjusted powerā data. The developers at both companies know that this is an issue that must be accounted for. Itās built right into both of those apps.
I feel certain that Zwift should have āaltitude adjusted powerā output for the avatars of people on their trainers at elevations above sea level. That would level the playing field.
How they determine a riderās exact elevation would remain a challenge, as would finding a method that prevents people from using altitude adjusted power to cheat in Zwift races.
But as it stands now, without having altitude adjusted power output in Zwift, as I mentioned in my original post, to those of us who live and train at higher altitudes, when I race against people in Zwift who are at sea level, it feels to me, from my perspective at 5,400 feet, that theyāre cheating.
I love Zwift and am getting into the racing side of it more, but the playing field doesnāt currently feel fair, or level, given that I feel pretty certain that people breathing sea level air have a significant power advantage over fully acclimatized riders who are breathing higher altitude air with a lower oxygen density.
At any rate, thanks again for sharing your own experience of the effects of different altitudes on your own power. It confirms what Iāve seen, and what all of my team mates in the Boulder area have seen as well.
Cheers 
Haha. Yes, thatās my age group. Colby is fast, for sure, but maybe not unbeatable. For me, itās less about actually beating the current record holder to get the temporary title of āworld record holderā.
Itās more about the journey Iād get to take to train for and then take a shot at breaking the record. That whole process is appealing to me. But even if I did set the record, I wouldnāt likely have it for long. Thereās always someone faster
Yep. Thereās always someone faster and no one is unbeatable. Colby probably would be one of the first to say that. Take a shot! As you said, itās more about the journey and process that it is the product.
As to altitude and Zwift, ultimately what I do for races & group rides - to the extent that I do them - is to participate in a category that matches my W/KG FTP for the location where Iām Zwifting. I donāt worry too much about whether Iām a B, C or D. Again itās the process and workout.
Here is a table from a study comparing altitude adapted athletes and non acclimatized athletes power output at different altitudes:
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This is awesome Henry!! Thanks for finding this and posting it!
I havenāt seen a comparison like this before. It confirms what Iāve seen in my own numbers when going down to sea level, training for a couple weeks, then coming back.
Thereās about a 10% loss when I come back home at altitude, which is exactly what this chart shows for my altitude here in Boulder.
And know I can see that after acclimatizing, thereās still about a 7% loss of power at my altitude.
Sure wish Zwift would add altitude adjusted power!
Thanks again for posting this chart.
There definition of acclimated is a few weeks not living at altitude.
Iāve lived at 7500ft for 10 years and my power increased about 7% when I went to Tucson(2300ft) to train this winter. Of course just being one person in a non laboratory setting isnāt scientific, but it seems to match up roughly with the chart I posted.
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acā¢cliā¢mate Äkā²lÉ-mÄtā³, É-klÄ«ā²mÄt
transitive & intransitive verb
To accustom or become accustomed to a new environment or situation; adapt. synonym: harden.
To habituate to a foreign climate; acclimatize: more especially (of persons), to adapt to new climates: as, to acclimate settlers; to acclimate one's self.
transitive verb
To habituate to a climate not native; to acclimatize.
I think enough evidence is there and Forrest is completely right.
I moved from 150ft to 7000ft (Puebla, MX) a year ago and consider myself adjusted or acclimated. Even though I train, here, there is no way to achieve the time I was Running at 150ft. Not Even close!!!
How can we get an official Statement from Swift?
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I agree, Thomas.
I think thereās enough data from studies, as well as enough of us that have personal experience with the power loss riding at altitude after acclimatization that Zwift should find a way to create altitude adjusted power output for riders who live above sea level.
If it required confirming our location by having our cell phone running zwift while also riding in zwift on our laptop/desktop, that would be fine with me. Or they could maybe use IP address location of our computer/phone thatās running zwift.
I donāt know how to get an official response from the Zwift team on this. I wrote them a support ticket months ago asking for someone to respond to this question and no one did.
I think weāll need to find someone who knowās someone who works on the zwift team to bring this issue up and get it addressed.
I love Zwift, but I canāt take the races seriously in zwift if Iām racing against people riding at sea level who have 10% more power than me just because of the air theyāre breathing.
The commentators of the Ineos team competitions that have happened on zwift are even aware of this, as they mentioned that several of the team riders were on their trainers at high altitudes and werenāt expected to be able to do well against their team mates who were at sea levelā¦and that turned out to be true when you watched the race unfold.
With as much as Iām riding in side now, because of the coronavirus lockdown, I would love to have them address this issue.
I wish someone from Zwift would respond to us in this thread, but Iām not sure they follow discussions in here. 
Take it into your own hands.
Can you not just drop your game weight a percentage that makes it about equal? The data on power loss at altitude is very solid and has been for many years.
Iād happily race against you with the extra 10% added w/kg etc, youād beat me with or without itā¦ but as youāre here asking how to do it legitimately you are clearly an honest racer.
Again. Until Zwift implement a solution, which will likely be yearsā¦ just hack the correct percentage and enjoy.
Iām sure nobody would have an issue with it.
Itās not real anyway.
Watching Alex Dowsett struggle with this has been enlightening. It needs to be addressed sooner than later.
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Whatās the source?
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Chart from The Effect of Racing at Altitude | TrainingPeaks
Using equations/modelling from possibly (because the training peaks article doesnāt cite properly)
And A theoretical analysis of the effect of altitude on running performance - PubMed
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I only see mention of several weeks.
for acclimatized athletes (several weeks at altitude)
There need to be study that look at the long term effects like Years.
I understand the effect of going from seal level to altitude there is a āshort termā difference.
But what about someone that acclimatized for years?
Respectfully, I doubt anyone here has actually read the journal articles or the studies that the above articles are modelling. By now they may well be outdated.
The website does say āBut they will only want to stay up for three to four weeks. Any longer than that and they will begin to lose muscle from riding at the lower power outputs. ā