The monolithic cats GM uses are pretty free-flowing...Unless you are making serious horsepower you don't need to contemplate a cat delete or upgrade...unless you have reason to believe there's something wrong with yours.
Wild guess I'd say a CAI with a K+N type filter and a performance muffler(s) might get you another 5-10 hp but don't increase the exhaust diameter, you'll loose low end torque and throttle response.....
I just busted out some 1.5" holes in the sides and bottom of my stock airbox and use a K+N panel filter (OE type) and it seemed to respond slightly to that at WOT...or could be it just sounds better, which it does.
If the stock exhaust would ever wear out so I could justify replacing it I'd try a Flowmaster 50 Delta series can or something similar on it...