Bkbandit, you are missing the point.
Right now, the Spit 16 is the "best" 'turny-plane' (much like the La-7 is the "best" Ho-n-Go plane).
So you (and some others, no doubt, so don't feel as if I'm coming after you personally) think "Oh, this Spit 16 is just to good! It can turn, accelerates well, has the best guns in the game, and isn't even that slow! Perk it!"
What Karnak is saying is that there is always a "Best plane".
If the Spit 16 gets perked (which it won't), the new "best plane" would be the Spit 8. The Spit 8 is a Spit 16 with normal span wings and .303s. They are identical other than that. So fine, lets perk that too. Whats the next "best plane"?
Probably the niki, I'd imagine. It'd go from 5-6% use to 14-15% (add almost all the Spit 16 drivers to it).
OMG, PERK TEH NIKI! Four cannons, absurd ammo load, turns on a dime and gives you back a nickel's change!
Good argument, lets perk it.
So now what happens? I imagine either the Spit 9 useage takes off or the Hurri-2C's usage does. Perk those?
See where we are going?
The Spit 16 does not enjoy such a huge performance advantage over the other "turny planes" that perking it would make any difference at all. The Spit 8 / Niki would dominate just the way the Spit 16 / Niki dominate a "furball" now. Perk the Spit 8 and Niki and you get Spit 9's and Hurri-2C's.