I just installed a Zard slip on. I like both the extra performance and the sound. The Zard has an easy-to-remove db killer, or baffle. It fits in the rear so the muffler does not have to be removed to change out the street-legal for the racing one. The racing one sounds like a big bad thumper. A little too loud for my overall comfort level. And I wasn't sure about the back pressure thing, so I mostly run it with the street legal.
It was expensive, however, I got it direct from Zard lab in Itally and the euro was down against the dollar. I like the Zard.