I see your point also, but I would like you to note that, unlike your examples of Me-109 and Ki-84, the higher boosts (especially the mid-level 66" rating for the P-38L, which was approved by the U.S.A.A.F., and the 72" rating for the P-51D) on American airplanes were quite common. Also, the higher the boosts are, the better the U.S. ships fare against their opponents, even if they're raised equally amongst countries (which should not be the case, as the highest U.S. pressures used seem to have been higher than those of other countries).
For an example, in the IL-2 series, the Me-109K and the P-38L running at their "official boosts" (1.8 ata and 60", respectively) are fairly evenly matched. The Me-109K at 1.98 ata and the P-38L at 64 Hg. MAP are slightly less evenly matched, but they're still in the same ballpark. Now, 64 inches is very conservative; an official memorandum has been found authorizing 66, and 70 and above are also well-known. Imagine a 72 or 75 inch P-38L. You can have the Me-109K at 1.98, I'm still going to be blowing past!
Ironically, I would probably have to ban the highest rated American ships (75" P-38L, et cetera) as it simply wouldn't be fair to the other aircraft.