Just another opinion out here, I think riding ability trumps the bike up to a point. There is no offroad sutuation where I would ride a 650 faster than a KLX. I think a pipe and airbox mods fix the "embarrasingly slow" stocker without affecting reliability and the factory 300 kit keeps the warranty. The carb improves throttle response, the big bore kits improve torque for lofting the front over obstacles. If you are jumping or weigh very much, stiffer springs are the next step after free mods and a pipe. It doesn't make economic sense to fix up a KLX if you want an off-road racebike.
But if you like to tinker and get a sense of accomplishment from mixing your own cocktail, the klx is a fun platform for doing it.