Your in luck, talk to green03 here. He has a tune for the 03s that is very noticeable I hear. Hi flow cats I wouldn't worry about. The flowmaster probably didn't do anything for power, just sound. If you want more power get a cat back, or get yourself some headers and have a custom made exhaust done from the headers back. Generally custom jobs aren't more expensive. Those two should do pretty good, you can add an intake if you want.
These trucks have 4 cats FYI. But I've seen ones go on ebay for 60 bucks so 250 will get you 4 flowmaster cats.