Lots of combat in AH takes place below 10K - I think the P-39's performance dropped off markedly above 15K. The P-39Q was a 376 mph aircraft with a 3750 fpm rate of climb - not a worldbeater but pretty respectable. In real life, jamming of the 37mm cannon was a known problem - but this would not be modelled in AH, so it should have excellent firepower for taking down ground targets and probably brutal snapshots in MA furballs, if the ballistics aren't too difficult to master. It's airframe was known for toughness and durability as well.
I assume lots of skins based on service from 1941 in at least 3 different theaters. No doubt the history buffs could find many with interesting paint schemes and nosearts.
In terms of number produced, years employed and its worldwide usage (Russia - North Africa, Italy, to South Pacific), the P-39 may be the most significant Allied fighter not yet in AH, I think.
