Just to start with a nitpick of my own, the Spit 21 prototype first flew July 43, the first production plane flew March 44.
The only Spits with a 5000ft/min+ climb rate were the Griffon engined Spit XIV (and some later ones, I suppose), and the Spit VIII/IX on 150 octane, which should certainly be perked.
A quick comparison of the Spit IX LF to the F we have now:
around 4000 built (350 of the current Merlin 61 engined F IXs built)
approx 15-20mph faster below 20k
climb rate around 4700ft/min, about 1000ft/min better than the AH spit
Overtake the N1k2 as the most popular plane? Perhaps, but it still wouldn't be the best plane in AH, and I doubt it would see the level of usage that the F4U-1C did. Besides, something has to be the most popular plane, and at least the Spit would be more representitive of the war as a whole, it was after all produced in large numbers from mid war on.
As to perking, I think calls to perk the Spit IX demonstrate how ridiculous the perk system has become. It bears no relation to historical accuracy, and now is starting to lose touch with the idea of perking high performance aircraft. The current Spit IX is the worst Spit IX variant, entered service in summer 42, and yet people are calling for it to be perked.