After our turbo is a pipe called the "s-pipe", which right now only iON makes a larger version of. Street Unit is developing one, and numerous other companies have promised them but haven't delivered anything. The Street Unit one is coming though, so that's what I'm waiting for. The greddy sp-2 is just a 2.39" catback. The corksport for $389, is a downpipe back exhaust I'm pretty sure, and is 2.39". By downpipe back I mean not including the s-pipe. The thermal r&d exhaust is a 3" catback. Apex makes 2.5" catted/noncatted and 3" noncatted downpipes (not s-pipes, the pipe after it), and 2.5" and 3" catbacks. The problem with the apex catbacks is that they go under the rear axle, so ground clearance is an issue on lowered cars. Essential Speed's is supposedly a turbo back, but if you send them money chances are you won't see anything back, so I'd advise against them. So the 2 options at the moment are iON and soon to be street unit's. Good luck.
~brian