THe slipon pipes alone does not really increase HP all that much, lets just say you will probably not notice the difference other than the sound. But i heard a story at one of the Dyno days at RMS, and there was two exact R1's one with Yoshimura full system, and one with Hindle full system,. The yoshimura system made one more peak HP over stock, thats right, one more HP. the Hindle system made 18hp more than stock!!! the yosh guy was pretty pissed. ANyways, it all comes down to the proper jet kit, or fuel injection kit. For exmaple, Powercommander, or the Yosh EMS system, they are both made by dynojet thou. An exhaust system plus a PCII can be a very good combination. But if you just have a exhaust system with out out a PC, then it is not used for more of its potential. just me $.02