Actually those speeds are perfectly attainable in-game, the IAS read-out on the speed bar has an error(ask TAGERT If you don't believe me) I tested many planes in game for speed and climb rate and P51's were among to most correct ones in-game.
The P51B and C do reach the speeds mentioned and even surpass it but not by a lot, actually by a very low %, while P51D is in same error at alt as is the 190D9 '44
Generally I most certainly wouldn't say either P51B, C, or D in game is either overmodelled or undermodelled
