I have Edelbrock headers, Borla exhaust, Magnecor spark plug wires, & True Flow air filter on my '03. HP gain is definitely noticeable but not neck snapping. And I don't know the exact HP gain in terms of numbers.
My advise is to stay away from Borla headers! Long story short, I had problems with it and Borla did not take care of me. I ended up taking it out and replaced it with Edelbrock.
For the exhaust, I only have minor installation difficulties with the Borla. But because Borla was such a poor company to deal with and the fact their exhaust is pricey, I wouldn't recommend it to anyone. See if you can use the search function at a later time because quite a few members on this forum are running Flowmasters and they seem to be very happy with theirs. Not to mention Flowmasters are very reasonably priced
I have not messed with the stock cat so I'll leave that up to someone else that has to give you advice
