So many variables. Do you have your KDX maxed out, or is there more room to move? Do you have pipe, silencer, reeds, sprocket, jetting, airbox cover, high flow air filter, etc etc? I find it hard to believe that all that wouldn't produce enough juice. Then again I'm a fairly conservative rider. I kept up fine with the 250 riders couple weeks ago on my 220 though. All depends on who you ride with and what you're doing I guess. I'm just not a fan of 250's on trails. Too easy to lose it and your ass gets sore on those skinny hard seats. Fun at times though, and if you're good enough then bombs away.
Don't forget the maintenance side of things though!! 250's have a tendency to break down back in the boonies, top end replacements, plugs and parts are expensive, etc.
seeya