Labelling an a/c over, or under modelled takes more than a measly 5 mph of a chart. Any a/c in AH that gets within that is close enough.
...Those charts do not indicate exact prop type, radio antenna type, intake snow guards? atmospherics, or any # of a dozen other factors not known, and thats not even getting into the flame damper issue. You need to know the exact details of the model in AH and compare it to an exact a/c in real life to comment on a difference of 5 mph, and even then, there could be reasons for a discrepancy, including the normal error range in any tests that were done.
The Mossie VI is as close to being modelled as its real life counterpart, as any a/c in the game. There is nothing I have seen that would make me beleive (so far) it needs any tweaking re: its speeds.