I'm running the Yosh full system.
I started out with the FMF Power Core IV slip-on and switched to the full Yosh set-up over the winter. The Yosh has a much stronger pull accross the RPM range. I never got past break-in with the stocker, so I can't comment on how much better it is over stock. But, IMO, the Yosh is an excellent choice. There are many good ones out there, however, like DSP and White Bros. I don't think you can go wrong with any of them. They all let that thing breathe better.