I have a hunch that the P-63 would be flown with a little bit lower standards than the P-39.
Usually, to have a successful flight in the P-39 (which is really just the challenge of getting back home, kills or not), you either need to be picking the crap out of a large group, and really using your E well, or only staying in the small fights.
P-63 could more than likely handle it's own in a large fight in the MA, but if it went under the long range icon as "P-39" it would probably always get the crap ganged out of it like what usually happens to me now whenever I do fly it the P-39.
I can't really say how well it would do in the MA's unless it were to be added and I were to try it, but that's just my basic guess as to how you'd see it in combat.