What Naudet, miko, Frenchy, Grunherz, etc. say. K/D means nothing out of context. N1K2 is the perfect ride (or the only real chance) to have some success at defending a capped field. This does not mean that a defender wont get killed A LOT (hehehe, know this first hand) but at least he will have a chance against vulch....ahem <cough>...honorable opponents, masters at the game of field capping

But the K/D of that plane will suffer a lot. On the other hand, you have the High K/D planes. Usually they are faster planes, the ones that you Know should have to take off from a back field, take time to grab, etc. 'cause they are unable to defend themselves unless they are fast.
On this whole issue, I would use Perk points as they have been used with the Quak....<COUGH, COUGH>....ahem!....(sorry)...with the F4U-1C

. Only to balance numbers. I know I've been very aggresive on the campaing to perk the F4U-1C. After seeing the results, I think it was right to perk it. I would like to see how it works to have this approach with the whole planeset.
Having perkies related with planes' performance is wrong. In the first place because performance is relative to flying style. A dedicated furballer will find Typhoon or La7 inferior to N1K2 or SpitIX...the opposite is true too.
If the goal of perkies is having a more interesting Arena, I can't see the performance as a factor to bear in mind. For me, an interesting Arena is a historical one. If it's not historical, it need to be as varied as it can be. Perks to have that diversity. Based on Arena Usage. That's all.
Cheers,
Pepe