Well I think this would be quite close to how it would perform...

633 kmh (393 mph) at sea level, 816 kmh (507 mph) at 10,450 meters on bubble top version, P-47J-Late. 628 kmh (390 mph) at sea level, 810 kmh (503 mph) at 10,450 meters in razorback version, P-47J-Early. Of course il2 compare is not completely accurate, the aircraft will only perform to how well the pilot can fly it. However it gives a pretty accurate overview of an aircraft's performance as you can't go and test at every single height etc... I've done some testing to see if they meet the figures with various aircraft and il2 compare is pretty dead on

These are all numbers for radiators closed also they are all True Air Speed figures, so not what your speed bar says as it reads Indicated Air Speed by default unless you have modded it.