[Holtzauge]

The compressibility issue the simulation shows is limited to the speed aspect and controllability is not modelled. The lower curve takes the compressibility drag rise and propeller efficiency loss due to speed and Mach effects into account while the higher curve assumes constant propeller efficiency and a "flat" Cdo which does not start to rise as the speed goes up. The problem is if compressibility is not modellled then any aircraft in the sim will build up too much speed in a dive meaning it will zoom too high in the pullout. This would work to the advantage of so-called BnZ tactics in the sim since energy retention would be higher than IRL.

Mach effects on compressibility are an interesting but complex subject but the C++ simulation posted does not model any impact on controllability due to Mach effects which is beyond the capabilities of the code which is limited to performance comparisons only. As you point out the P-38 was especially susceptible to this phenomena but the Mach "tuck" effect did affect the other types as well and it was more of a question at which Mach the problems started to appear. However, I think modelling this is even more complex and I think it would be a significant step in the right direction just to get the flight performance aspect of compressibility in to begin with.

While it was some time ago I looked into it, I still believe that the Me109 elevator control modelling in IL2 was off because it modelled loss of control authority more due to TAS than IAS: At low alt you were OK at certain IAS but at high alt you lost authority at the same IAS even though the Mach number was quite low. Since I have not flown the Me109 here in CloD I have no idea how this is modelled. Hopefully it has improved.