Hi, I have tacx booster. According to the setup instructions I have to manually configure my trainer to difficulty “2” which is really pretty low. When riding with Zwift, the maximum estimated power is around 400 Watts. So my question is: is there any way to reach a higher estimated power output with this trainer? Can you add a power curve for a setting 4 maybe 5?
Unfortunately there is nothing that you can do to get a higher power curve.
The best is to either get a power meter or to get a different trainer.
If they added new power curves for heavier gears, this is possible 
Yes if they add the power curves. Zwift actually did tests on these trainers to calculate the power curves.
I would not hold my breath for more power curves. That has been asked before. but we can always hope.
Ps, I highlighted your request in bold so it does not get missed.