You may have to go up 1 maybe 2 jet sizes on the main. On my current setup I went up 1 on the main. Hopes this helps.

 

Does the 400ex kit come with a headpipe as well?

 

The ad in Dirt Wheels for Rocky Mountain ATV has one listed for $179.99. I'm assuming that's just the silencer, for the 300EX it says includes header pipe, for the 400 it doesn't. Supertrapp has a web site at www.supertrapp.com try there and see what you can come up with. Good luck.