With our GV all I did was buy a 2" in/out Magnaflow muffler (absolutely straight through, no baffles, chambers, etc to mess with the exhaust) and have a local muffler shop put it up there in place of the factory garbage can of a muffler. I beleive the factory pipes are stainless steel and they are pretty well setup. We got a noticeably seat of the pants increase and it's got a nice pleasant growl to it now.
Something similar would cost you half of what Calmini's crap costs. HINT: do a search for Calmini, exhaust, and muffler. Their systems don't seem to be made that well as I have heard several comments about them rusting off in just two or so winters. Might go longer in warmer climates.
Not that there are many bolt on engine mods for Suzukis anyways but remember that other than a freer breathing and exhaling engine there isn't much you can "bolt on" to make big power short of a turbo or supercharger.