P-39 is the individual top scorer allied plane against the Luftwaffe during WWII , people often forget this
Bad feedback about P-39 made by British and American pilots were about the first series of Airacobras in 1941 , if you look at their griefs , most of them were addressed by Bell in later variants except the poor performances at high alt due to lack of supercharger of the engine ( If only the P-39 had been fitted with a supercharged engine , it would have been awesome )
In game , the P-39 quite accurately reflects its real life counterpart in term of performances at low to medium altitude IMHO (eastern front conditions ) , it is still inferior to anything German except the G6 , and still very poor at high alt as it should be .
i'd even say perhaps its responsiveness at higher speed above 400kph is slightly undermodelled (according to accounts i've read , not only Golodnikov but in various other books , Soviet Cobra pilots had no problem following 109's whatever they did except in sustained climb, they could fight on par with them in the vertical plan and out-accelerate them in a dive , not sure we can reproduce this in game )
Also the damage model is quite frustrating ( any hit in the rear part of the fuselage necessarily hits the engine which is annoying , due to DM simplification ) . In our sim Il-2 , the 109 and the P-39 are the planes that have been given the most attention in FM , can't remember where i had read this , must be a few years ago on ORR .