what is the code for the O2 sensor? you need to know if its the rear O2 sensor or front O2 sensor that is causing problems. replacing the sensors after the cats will do nothing for you performance wise but if the front O2 sensors are malfunctioning then you will definitely feel a difference and most likely get better gas mileage since they are crucial for fuel managment.

if you dont understand what is going on then you should go see a mechanic or chevy dealer.