I'm not sure if it's because there are more of them around, but to me, it seems the Spit 16 is easier to kill than the other Spits. This is from the perspective of flying P-51B's, C.205's, and Bf 110's. I was thinking it might be because the Spit 16 doesn't turn as well as the other Spits -- and, yes, if everyone flew to the strengths of the Spit 16, it might not be easier to kill. However, a lot of people just get immediately into stallfighting with any Spit.
As far as perks go, it depends on what the goal is. If the goal is just to keep planes that weren't flown much in WWII not flown much in AH, that's one thing. If it's to promote use of a variety of aircraft, that's another thing and not how perks are working today. For the later, you'd need to adjust perk points adaptively, adding perks to more-popular planes.