I also have a 02 KX250 and agree with mifun3, smooths out the power band, runs crisper on the bottom and has better response down low but no real power gain.
If your looking for more low end punch for tight track / trail riding then switch to the FMF Gnarley pipe and the v3 (my favorite combo).
I use the v3 reeds with the pro circuit platinum pipe and silencer for desert and long sand tracks.
Either way you can't go wrong with the v3, I'd buy it again.
Mine is also stock except for the pipe & v3.
Paul