I believe... for best results... get ahold of a set of stock headpipes from a 2006-2007 model to get rid of the cat. (I have a set with 500 miles on them that I would sell if you like) Then get an 06-07 slip-on that fits your fancy. Getting rid of the cat (IMO) will be your greatest performance gain. Akro, Leo, and Yosh are all high quality parts.
Don't know about 08/09, but I've owned quite a few for 06/07 and have seen a few others. From my experience, both best quality and fit/finish is Akrapovic, but I haven't dyno'd it. Not cheap though.
