I want to replace the stock muffler on my 35hp. I want something a little more durable and rust proof (stainless). At this point I don't plan on doing anything else to my motor. Realistically should I expect any gains from one of the perfomance mufflers available? If so what should I expect? 100 more RPMs or more? I also run my motor at an elevation of 4500ft above sea level. Will that make a difference. Thanks in advance for the help.

Lots of options, BPS versions, Marshmellow, EVT.

Maybe a little RPM gain, try a K&N air filter in the filter box to improve air flow in while the muffler is helping it get out.

Will help carrying a load a little better.