I like to look at it as potential vs. reality, and reality should reign supreme.
If we look at potential, the K4 and D9 should both be perk planes.
In reality, we both know the number of people that can dominate in them is small enough that a perk price is completely unnecessary.
By contrast, the Tempest has both the potential to be a perk plane, and in reality, is quite easily dominant in the hands of many, many players. As such, it gets, and deserves, a perk.
I think that the c-hog has that potential and reality, though I could be wrong.
These are exceedingly poor examples.
Neither the K4 or the D9 is clearly the fastest unperked plane at typical MA altitudes. Many planes are very near both in speed. Neither is a great turner. The D9 is in fact almost the worst turner, beating only other 190s in that regard, and out-done by quite a few types in climb and accel. Its gun package is good but not uber. The K4 is quite the dogfighter in the right hands, double superior to quite a few models in fact, but boasts numerous serious weaknesses: poor dive handling, poor views, squirrely handling, and what is among the most difficult gun packages in the game.
The Tempest, by contrast, is the fastest prop job at typical alts, out-accels the whole field, carries the best a2a gun package in the game, and, here is the clincher, actually can out-turn the P-51Ds, P-47Ns, D9s, etc, that come nearest to being able to run with it, although most Tempest fliers make no use of this potential. The singular exception to this rule is the La7.