A cat back really doesn't do that much on it's own insofar as performance is concerned. Sure you may open the flow up a little with the right mufflers and what not; but that's about it besides maybe shaving a few onces or pounds off. Your main increase in punch will come from a high-flow cat (remember, a cat back does not include the cat) and a good set of headers.

The thing of it is is like I said previously, these cars are pretty tight anyway. When you go to do mods, it's an all or nothing type of thing. Sure you can piece-mill it; but it's best to do everything at one time if you can. What you do first or do at all is really up to you and your budget. ;-)