Most of my experience is with the D model 39, the D does a couple things well but it going to be a real challenge in the LW main arena. The D will pick up speed in a dive quite nicely (it needs to it doesn't start with much), and I feel like it can make some pretty sharp turns early in the engagement. I say early because most maneuvers are going to bleed energy and the P-39D is very slow to rebuild that lost E. So start from the top of the fight and work down, when you start to get near the bottom get out. Try to leave yourself a couple thousand just in case because putting the nose down is the only way to quickly build up airspeed in case you need it.
The Q is a much more capable airframe, with 10-20mph better speed and an extra 1k/min climb advantage up to about 10K thanks to WEP (according to the charts on the wiki). I do find the armament somewhat problematic, being a relatively poor shot the 37mm is a difficult weapon to hit with, low muzzle velocity, and low rate of fire. What that means is I have to get in really close to score a hit, the D model has the 20mm option which I prefer despite the small ammo capacity. I don't fly the Q enough to have a good feeling for whether the increased firepower of the wing mounted .50's is worth the drag and weight penalty. Right now despite my issues with it I think it's a better idea to go with/out the gun pods but I'm happy to hear from more experienced Q drivers.
Despite all it's issues I really do enjoy the 39, particularly the D. It's a real challenge to fly in Late War, and I love the history of the P-39 in 5th Air Force service in New Guinea which keeps me flying it.