The rev box is not going to help him. The 400ex does not make any useable power after the stock rev limitor with just the 87mm piston installed. Once you add a new carb, modify your intake, add a free flowing exhaust and port, then the aftermarket rev box will be needed. Check any dyno report on a 400ex, and you'll see what i mean.

 

Ok here is one of the things that I found out. I ran the bike on the dyno to see where I was at and max HP was about 26.7 HP. This is lower than I thought it should be so I decided to double check the valve timing. I was advanced one tooth. I would suspect that this would cause a difference in performance but to what extent I don't know. I will try and get the bike over this week to run it on the dyno again before I put the cam in it. I will also post detailed dyno results when I get a chance. Thanks for all of the great informaiton that everyone has been providing.