Replaced the stock exhaust with the HKS Carbon-Ti catback. Pretty much a direct bolt up except you have to shim the hell out of the spring bolts (Don't re-use the springs). Take up the extra slack created by the spring removal with a spacer over the bolt, or just washer it up.
Better breathing (Not a whole lot, it's only a catback) up top. A little loss down low...so just don't get caught down low (< 2000 rpm).
Love the sound. Low gutteral burble, nice at WOT as well. No droning observed.
One thing you may want to consider though. The Carbon-Ti is the same as the Hi-Power from HKS. The Hi-Power is cheaper. You could use the extra dough to by a down-pipe and run more HP than those of us with the Carbon-Ti Catback system