The factory exhaust has some pinches in it, but a catback doesn't really give you much HP or TQ unless you are puting a pretty highoutput head/cam or forced induction on.
You get way more gains from a header and high flow cat install.
With a stock 04 manual I made 307HP and 327TQ, at 500 miles, with a LPE intake setup and Corsa touring catback.
With a tune, LT headers, and highflow cats added to the above, I'm at 330HP and 349TQ.
Also watch out for those claiming big gains with tunes. What they might do is compare a 1st dyno run low number to their tuned high number. I see a 5 to 10 HP and TQ gain on a 3rd dyno run compared to the first (Reguardless of stock tune, aftermarket parts, and custom tune). I've had my car on a dyno 4 seperate times to compare changes and the first run is always a low number and they go up to the highest on the 3rd dyno run. The forth sometimes drops maybe 1 or 2 HP.