The issue with Wing shedding in the P51 in IL2 under G is that you are exceeding the 13.5G global G limit that applies to all aircraft in IL2. In the case of the P51 the in game "light" elevator lets you pull 13.5g with relative ease, more so than most aircraft hence its propensity to pull wings off. Perhaps some high speed cement is required in the P51 elevator circuit.
In reality design structural G was actually lower than that in the order of 10-12g for most US fighter types.
As to P38 compressibility issues below 10,000feet. Well its still possible if TAS is above 460MPH. P38 Operating handbook Page30 illustrates this quite clearly. Exceed 460TAS at any altitude and you re in the realm of compressibility affects in the P38.
Testing the P47,Tempest and D9 I get Mach 1.15 in all of them before bits start to separate. I think this could easily be fixed.