I’m going to comment but I’m not sure what I have to offer is directly relevant to the OP’s question.But here’s my take…
I use a Kickr and have trained and raced with power meters for over ten years.
Once a spindown has been performed, the Kickr creates a similar “feel” and output as the various PMs I have at hand. Using Trainer Road I have no fear of being misled. I can and have done an hour on the Kickr and TR and then gone out and done an hour in the real world with a powertap and both efforts numbers have been really close or basically the same.
I have noticed the extra speed in Zwift too. I’ve input my real weight and have the “Trainer Difficulty” setting at max. I have given this extra speed a good long think and maybe it is accurate for Zwift, after all it is, again, not the real world.
But we are trying to create a simulation as close to the real as possible so I would like to figure out a way to get the speeds down to what I consider real world speeds. Maybe I need to add a few pounds to my weight and take trainer difficulty down a notch or two. Maybe.
Zwift seems also to over report my wattage a bit but it is difficult to say if this is true too. Indoors, I tend not to hold back. Using Zwift, I can just bury myself and just step off onto the couch. I can’t do that in the real world. In the real world I have to get home. Lol. So maybe that’s all good.
But here is the interesting part; When I compare my Zwift ride data with rides done out in the real world they are very similar.
The Zwift rides have higher peak power output numbers but overall they have about the same TSS, average watts per time periods, etc. That extra speed shows up as greater mileage. NP and IF are a bit higher due to the higher peaks but I think these are valid-ish numbers. Like I mentioned, I find it far easier to damage myself on the trainer then I do outside and NP and IF reflect this.
So am I comfortable adding Zwift data to my outdoor training data? That’s the real question and I am still thinking on it. For now, the rides I do on Zwift are just to keep some pedaling in my legs while I’m stuck indoors. These numbers aren’t all that interesting - it’s just volume. But in a few weeks the numbers will be important and hopefully by then I’ll know if I can trust the Zwift numbers to reflect the outside world enough to add then to WKO+.