It is futile to try and determine what formula HTC uses to put what aircraft where in the ENY scale. Same goes for which aircraft have a perk price applied and those that do not.
One could safely, I would think, assume that usage of an aircraft has little bearing as to the ENY score and/or perk price applied to it. If that was the case, the Spit 14 would not be perked and the Spit 16 would be.
I mentioned the "LESS" than 5 ENY applied to the aircraft I mentioned simply because of their current ENY ranking. Mind you, I'm frankly a bit confused, like many AH players, at how the ENY scale and the perking of planes is formulated (I bet there isnt one, just a glance or two plane performance abilities and actual game play stats).
Im glad to see the P51D is finally a 5, and not an 8. Name an aircraft that is as versatile as the P51D? It is amongst the fastest at all altitudes, it can carry 2900lbs of ord, and still has plenty of ammo and firepower in the 6/.50 cals. Dont forget generous climb rates, a dive ability second only to the Thunderbolts, plenty of maneuverability, great pilot visibility, and the ability to soak of a good amount of damage.