I use the HKS HI-Power 2.5" catback. I love it. I previously had the Apexi Megaphone dunk. I loved that too, but I like the style of the slant HKS better.
By the way, RS and GS exhausts are different from the GST ones. My Dunk exhaust went over the rear axle section or whatever you want to call it, and that was for the non-turbo. The new HKS is from a GST, and it goes under that section and is very low. I love that Slant though!