Flown the P-39Q a lot in the MA. Its a sweet ride. In clean configuration, sans gondos, it is a very competative dueller below 15k. It will stick with most fighters in a turning contest, has combat flaps, and it has a very good, stable dive. The armament is better, imho, than the Yak-9U or the Yak-9T, as the 37mm does not fire too fast so you dont waste ammo. A few hits from that nose cannon, and its goodbye.
With a DT you can get 40 minutes endurance in the MA. Sea level speed is 329 TAS in WEP with no gondos, so its no slouch.
Not a brag, but most of the 1 vs 1 against N1K2s in it I have won. So it can't be that bad for manueverbility or power rating, and some were vets, not just n00bsters. Its not "uber" but you have a chance in it, and thats saying something.
Its very under rated, why more dont try it I don't know, I think a lot of players get stuck in the habit of what they roll, but each to their own.
...The P-39D can be effective, but with 300hp less in the engine, is not nearly as competative, more in the league of a P-40E.
Its a very fun, and capable ride.