What are you ideas of what i could do to a stock 400ex? Right now i'm lookin to get the White Brothers Pro Meg pipe, k and n air filter and the jet kit. How much would that do. How much low end, mid range, and top end would it give. How many mph would this improve it.

 

Stop worrying about the darn top speed already.

 

With the mods you listed,you will notice a big difference in low end power and a moderate increase in mid power.Top end should increase about 3mph.If what you want is top end, put a 426 kit in it and then gear it higher.You will then have a good low end and a good top end..hope i helped