Caution--somewhat long:
I always though "unbalancing" meant aircraft performance--that is, those planes which utterly outperform the "normal" planes would be perked. That means planes like Tempests and ME-262's and such are "perk" material. By this definition, a plane with average performance (F4U) is not really a "perk" plane. When I subscribed to AH, the F4U-1C was common, but no superplane. I would much rather fight a horde of F4U-1C's than a horde of N1K2's or Spit 9's, much less Tempests or jets.
I don't think a plane with 20% of the arena kills is "unbalancing". Rather, I think that's a fact of online life. As long as the planes perform differently, one will be used much more than the others. This is the case in every game I've ever played, be it FPS, RPG, RTS, or Sim.
Actually, 20% for the most popular plane/item/weapon shows EXCELLENT balance--usually the number is more like half!
The only sure way to balance plane usage is to limit total numbers of any plane allowed up at any given moment--which clearly wouldn't be very popular (and I myself would hate it).
I still fail to see the benefit of perking the F4U-1C. The numbers will simply move to some other plane. They WILL NOT equally disperse among the other available planes; instead a majority will move to the one with the percieved advantage. No matter what, one plane will see a dispraportionate amount of use.
I do see RAM's point--it isn't "balanced" in one sense of the word when one plane dominates the arena. And it does get old fighting the same plane over and over (this is why I wouldn't like a Historic Arena). However, I think this sort of "imbalance" is an unavoidable part of online gaming. I also doubt it's what the "perk" system was originally meant to address.
A more productive solution: Develop tactics geared spcifically towards the destruction of F4U's, and fly accordingly. STAY AWAY from those guns! DO NOT take the chance of letting it get a snapshot! Make sure you have more distance than normal when trying to rope them! Negate their advantage.
In a nutshell, I don't think the F4U-1C is "unbalancing" in the same sense a ME-262 is. That is MY opinion. Whether the C-hog is fair or not all depends on your interpretation of the word "balance". Being human, we won't all agree.
On a second thought--what really matters is HTC's defination of "unbalancing"!!!

J_A_B
(and since I don't currently subscribe to AH I have no secret "agenda")