400's fall more in the "fun zone" where you want to race for fun and not competition. if you want to get competitive you will either need to build a monster 400ex or buy a 450. i recommend buying the 450.
The rider is more than the motor. Put a 426 kit and slam that beast to the ground. Stage 2 cam, intake, exhaust, and a CDI you will be able to hang. Now if the 450's have the same upgrades minus the 426 you will have to be on top of your game.
^^^^^ that is the most true thing thats been said, if i have an off race ill be lucky to hold onto third, you can do as red said and go 426 and stage 2 cam or you can put a stage one cam and a 440 bore (ive seen somewere that a stage 2 wont help you out much with a 440 kit but if im wrong please tell me i would love more power)