I think i took up the Jak9T only a handful of times, my experience with it is very short.
What i noticed: the ballistics are awesome. You can hit a straight flying, tipical escaping target from 800+. In a turning fight i couldnt hit crap with it, couse of my habit of leading way too much (as i should aim with the 151/20 and the 108). Its fire rate is pretty slow, in the crossing snapshots the opponent can fly through your bullet stream and remain unhurt. Your ammo load is very limited, you cant spray 10 rounds bursts like with as the mk108. Also the plane itself is slow and is a poor turnfighter. I would say, practice a lot in the DA til youre confident.
The p-39's gun is a waste. Poor ballistics, horrible fire rate, very small ammo load, also the plane is next to be inpotent too. Its only for masochists.