The misconception here is that some people think that if the FM of airframe "X" is changed, then it has been checked against every other aircraft in the sim for relative performance comparisons. Not so.
The HSFX Expert Mode FMs are established against historical flight performance data. Correcting, for example, the Fw-190A4 may push its own performance much closer to reality. But it could still place it at an unhistorical advantage or disadvantage against other aircraft in the sim, and the reason for that is that not many aircraft in the sim have closely historical flight performance anyway. The multi-engine aircraft were particularly problematic in early IL-2 versions.
Given the 600 airframes now supported in HSFX, it will take a LONG time to get them all updated to near historical performance. The guys who do this work for HSFX are trying to build reference FMs, and they work hard to do so.
In the meantime, people who are happy to accept the stock FMs (warts and all) can just turn off the Expert Mode.
Cheers,
4S