I say go with the stroker! 465, 440, 452, whatever, they all rip
I was in the same boat as you while using a basic 440 big bore kit. I was down on oil and overheated the head to the point that both my intake valves seized (you know what happens to them after that )
That was a perfect excuse to step up to a full-on GT Thunder stroker setup that you can see in my sig. It was an excellent decision IMO and is as reliable as stock It is very hard to convey over the internet just how much power my engine made after the rebuild.....one ride and you know this ain't your run of the mill 400EX motor
Do what fits your budget though. If you can afford it, stroker setups can be a sure route to stronger performance. If it just isn't practical for you, then I would possibly look into a used head from eBay. You could run that for a while and maybe sell the 400 for a 450 quad down the road.
00' 400EX - GT Thunder 450 stroker, custom JE 12.5:1 piston,
+7mm stroked crank, HD stroker rod, HRC cam, GTT full race porting, +1mm intake valves, FCR 39 carb, CRF timing chain, Denso IXG27 iridium plug, full Hinson 8 plate clutch, Desert Toyz Supercooler w/ SPAL fan, UM airbox w/ UNI, CT Hi-Output exhaust, Roll LOBO II F/R, LT PEP 8 click ZPS piggyback fronts, PEP ZPS rear, YFZ brakes/450R master, CCP stabilizer, Hiper Tech beadlocks, Durablue X-33, TAG, Quadtech, ASV, AC, IMS, RAD, Works Connection, Galfer..
05' Kawasaki KLX 110 - Fast 50's springs F/R, HRC cradle, Rox risers, FLY fatbars, NGK CR6HIX iridium plug, Twin Air filter