IF you've got a turbo, you kinda need a turbo back.
Obviously the turbo will alter the header, so only getting a cat back won't work (the downpipe wouldn't fit onto the turbo). An aftermarket catback would also restrict your engine, since they're 2.5" and the turbo is pushing out much more gas than an NA car. You'd need a custom 3" turbo back exhaust for the best power.