I don't argue historical rarity.
The 152 is superior to the 51D, D9, and F4U1A. Not clearly, but it is. Part of the reason it's not clearly so is because of how deep into the envelope, and consistently, you have to reach to have a sum performance that's really perk worthy. But once you have enough stick time in all three, it's clear enough. The F4U is way lower. The D9 would also be a complete minor to the 152 except for it's speed. The 51D is lesser only because of its lack of acceleration; the 152 can just leave it behind (below) from low speed -- but overall the 51D is only lesser by a really small margin, and it's so much more eminently user-friendly that in practice it's clearly a better plane for the average player.
So, like others said, you can't realistically have an ENY system strictly decided by either potential or popularity. It has to be a comprehensive combination of both. There's another thread right now with this topic, so I'll leave that for that other thread. I think the chog is definitely on target with a price of 1-5 perks.. The 4-Hog really couldnt be free. Anyone that manages to keep the F6F or F4Us in the air long enough to get just a couple of kills, or dodge a few more attacks, would benefit from the 4-Hog's bonus in performance. It's not like the 152 vs 190s where it's really a different animal. That bonus in performance that makes the -4 the best prop dogfighter in the game is easily accessible.