Why can't you make short work of all opponents in a P-39Q with a 37mm?
...The 20mm option gives you 60 rounds of ammo, its hardly a great trade for 30 rnds of 37mm?, one hit of which will blow a fighter into flaming bits. I have not used the gondolas once in the MA with the P-39Q, just the nose 37mm, and the two 12.7s. Its all you need.
The call for the 20mm is "the grass is greener" stuff because we don't have it as an option on the Q. Thats all.
As for the P-39N, we should have that one, since its so easy to add, although for me, I would prefer the P-39Q, as I dont consider the weight of the 30s and ammo in the wings worth it (neither did the Russians, who removed them).