Stock makes the most lately , if you want noise go buy some pipes.lol. maybe removing the cat and doing a 2 into one pipe with fuel programmer would make some power. my Rotax 800 had a 2into 1 pipe and it sounded like a small v8 which was fun.
I think it depends on who you choose to believe. IMO most if not all of the aftermarket exhaust system's will give you some performance gains just simply due to the system removing the Cat converter. Muzzy claims about 10 HP and many people on here don't believe that claim. A few other people have dyno'd the muzzy and have gotten different result's. I have purchased many muzzy exhaust's and have always gotten good results and I love the sound. So I purchased the muzzy from a vendor I trusted who dyno'd the muzzy and said he saw an 8 HP improvement. Once it was installed, there is a definate improvement in throttle response. So like I said, it will depend on who you choose to believe.