I pretty much agree with what QM said; usually not a significant gain, though a few hp is not unusual, but that is barely noticeable. Plus, on a naturally aspirated motor (non-turbo or supercharged) the optimum tuning requires some tuned backpressure/scavenging and a totally open/free flowing exhaust will actually reduce low end torque. I really like Burns Stainless stuff too, if a local shop or you can put together a good fit from what they have
http://www.burnsstainless.com/