With a few exceptions, most of the '01 catbacks were the big restriction in the exhaust system. You would be better served getting high flowing mufflers welded in IMO. If you could find a '99 catback you would possibly have better luck.
This very test was performed at a dyno day last year at Mustang Magic. A used freebee 01 catback picked up 5 Hp over the factory 03. If you can get the 01 for free, what the hell, throw it on....if nothing else it will sound a hell of a lot better and give you 5 free hp.