I had one on my TT .42GP and got maybe 100 r.p.m. more after I drilled out the opening. The performance was about equal to the mod. stock (drilled tailpipe,radioused the inside of the tailpipe opening, and streamlined the baffle) muffler, but I think it looks a lot cooler[8D], and as mentioned, it's lighter. Both gave about 200-300 r.p.m. increase over the stock un-modified muffler...