The stats really just mean that many people who fly it don't actually know how to use it properly.
Again, you can say that about many, many other planes in game. Actually most planes stats suffer from people who do know how to fly them properly. There is no reason the Spit 14 has a proportional higher number of pilots not being able to utilize it's strenghts than other planes - The fact that it is already perked does keep the ever crashing 2-weekers out and usually results in some more cautious flying (don't lose that perks!) by the other pilots.
And let's not forget why the concept of "perk" was introduced at all:
The perk system is a way for HTC to introduce some interesting but otherwise unbalancing planes on a limited basis but the benefits go deeper than that. Perk planes (and vehicles) would be things like Me 262s, Ta 152s, Tempests, B-29s, Ar 234s, Tiger IIs, etc. These are interesting rides but would be very unbalancing if they were available on an unlimited basis. So there won't be unlimited availability but they'll be available as bonuses or perks every so often.
It's about the effect the plane has on the MA. And yes, looking at the Stats is a very valid way to examine how a given plane is having on the MA's. And it's a quite objective one:

Perk fighters are marked in red, blue is the now unperked Ta 152. You also should bear in mind that many planes with even lower K/D do suffer from the fact that they are used for a lot of ground work - bombing CV's, porking fields and other suicide runs. This is not the case with the Spit 14, which is a pure fighter without any ord loadout.
Some absolute numbers from that year
F4U-V 123,000 kills
Tempest 84,000 kills
Me 262 49,000 kills
F4U-4 43,000 kills
Me 163 19,000 kills
Spit XIV 17,000 kills
Note that the XIV has even less kills than the highly restricted Me 163
The Spit 14 is very comparable in pure performance to the 109K. Yes, on paper it is better in some aspects, but that alone justifies a lower ENY, not a PERK status.