The key to getting the KLX to pull it's front wheel on demand is a pumper carb. I just put a Mikuni 33mm on my 300 and it really woke the bike up, stock exhaust and all. And regarding 400cc being the magic number, I suggest you read one of the articles on PlanetKLX that compared the XR400 to the KLX300. The conclusion was the power to weight ratio is about the same and the KLX handles much better. Unfortunately the KLX also had some nagging quality issues which you have to go to Stroker to fix to this day (they're cheap, but why didn't Kawi do it themselves since Stroker has made them aware?!!).
Then again, the YZF/WRF and KTM's are also light bikes and have 400-520cc, so they have raised the bar. Frankly, the bikes the most behind right now are the Honda XRs, with their air cooling, heavy weight, conventional forks, and SOHC motors. But they are extremely reliable. And we'll see what happens with CRF technology through Honda's line up.
I got a good deal on my bike used with a rebuilt motor since the previous rider had drown the bike trying to cross a river. I do know the 300 is a far better bike than the 250 but a pumper carb is a great thing on either one, as are the free mods such as uncorking the exhaust and removing the intake obstructions (again, see PlanetKLX).