Newbie questions (any feedback will help):
Say i were trying to get 425rwhp from a 2003 Z06...
1) would it be worth it (any significant gain in power) to spend $$$ on an aftermarket catback?
2) Other than headers, CAI, Cam, pulley, ported TB, st shifter, and a tune... what could i upgrade to get to 425rwhp? would a small cam (no larger than a 224 since i have to pass the cali sniffer) do it?

I'm debating whether to buy a 2005 GTO or C5 Z06 thus the questions... thanks in advance!

I would say probably not. I believe the stock C5 Z06's lay close to 340 HP stock to the wheels (someone may correct me). From a slightly better cam and catbacks your probably looking at maybe 50 or so HP after a tune (+-10HP). So I'd say you need a little more. To the question, Catbacks are GREAT when combined w/ headers. The gains arent worth it without them in my opinion. Full exhaust is the way to go.

most dyno pulls have the 04Z06 at 355rwhp. Don't know if they changed from 03-04. I know headers, exhaust, and a tune will net you decent numbers.

A 75 shot would get you there all by itself if you wanted to go that way and it would cost you less than a set of Long tubes...