I just put one on my '05 CRF450R. And it seems to be working quite well. Hard to tell exactly, with the temps what they are this time of year, upstate NY. Throttle response seems to be a bit quicker, less bogg too.
I did put a Boyenson on my KX250F, $$ for $$ I think you'll like the WIRKS version a whole bunch.
I also put one on my 250X. I did all the other mods at the same time. I got to ride it for the first time yesterday. I can't tell if it isthe sum of all the mods, but I put one on when the carb was off just to be safe.