A high flow cat is one of the first 3 basic mods that most of us do. 180 thermostat (drill one small hole in the flange), high-flow cat, and some form of intake or drop-in performance filter. These three will wake your car up in terms of throttle response.
The high-flow cat with your miles would be a good place to start. If your factory cat has alot of miles, you can regain some lost power, and then some. After about 100k or so, I'd bet a cat replacement would be more obvious to the seat-of-the-pants dyno.