I may bolt on a header and forget the catback (for now). My '04 has pretty good stock exhaust piping with decent diameter. I also don't want a ricey sound from a loud muffler, and I have yet to hear a 4 cyl. catback that sounded good to me, except for a WRX STi I heard (which made me drool). I've seen some V8s with pretty restrictive and smaller exhaust piping than my car, which may explain why new piping makes so much difference with them.

Since exhaust has the greatest velocity as it comes out of the head, it would seem to me that a more free flowing manifold (or header) would be of greater benefit than changes downstream. (A slow exit slows down the rest of the trip) Also, couple that with the fact that just getting exhaust gas out of the cylinder more completely would lessen dilution of the next intake charge. I could see a header alone gaining performance. I would agree that using it in conjunction with a catback would do even better, but how much depends on the car/engine.

I've read plenty of comments by people about how much difference a header made, but not as many raves about catbacks.