It's the cannon's, those Hispanos are very deadly any 4 Gun Hispano plane is a killing Machine, the Huricane & Typhone are extreamly deadly, It is also the huge ammo load the C caries that helps, and as you say the stats are effected because of it's use as a vulch machine.
The curent Gun model favors The 50 cal and the Hispano, thruth be know they othe cannons in the game are typicaly underrated relative to their historical effectivenss, as are other guns which used a chemical engery componet, the MG 131, the Breda 12.7 and the MG FF and Type 99 MK I for example, but all of this is realy an argument almsot about what once was, since this is all changing in AH2, the gun model is being changed.