A buddie of mine just bought a 00 400ex. Of course he wants more speed out of it already. Here is what he wants to do. Install a big gun exhaust, rejet, rev box, and put in a k&n filter and remove lid. My question is do you think he will gain 5 mph on top end, that is what he is shooting for.

 

Even though the BIG GUN pipe is not one of the better high rpm setup's, he should gain around 4 or 5 mph on top end. This is assuming jetting is exact for his mod's (which is critical). If his only interest is a top end gain, he could go a tooth up on his front sprocket and probably get the same gain.