What kind of mileage are you getting? The best thing I can suggest it to figure out how to drive it for the best possible mileage, where the engine likes to run, etc... If I'm driving purely to get the best mpg around town I can get 18mpg. That's pretty good for a 4000+lb car with a v8. My old gto only got about 16.5 no matter what I did.
*EDIT*
I guess I missed the part about using the "better gas mileage" as an excuse to buy after market parts... A good exhaust should help in the mpg dept.
