Here is my personal opinion:
If you want to do "normal" off-road you don't need a big bore kit on the klx. If you do all the mods (exhaust, derestriction, bigger air intake, smaller gearing...) the bike will get you anywhere you want the same way other dirtbikes do.
Now if you want a racing bike, it's really pointless to pick the klx in the first place, because it's main issue is not the power, it's just bike is not design for extreme enduro. My biggest issue with the bike is when I start to go fast on big rocks or so the bike feels very clumsy (because of the suspensions, weight,...). The only issue with the power is it's hard to get the front wheel up (except in first gear), and a lot of shifting with short gears.
If you want to do proper super motard then again pick another bike a 250 is just not enough on the road.
I think people don't get that you don't need 50 hp to go off road... most of the trails you don't even have enough traction to use that power, the most you will go on dirt tracks is 100km/h and that really fast already.