Put the biggest nastiest cam made in it if you are going to swap to a loud exhaust. I never liked the sound of a stock engine with loud exhaust. It needs a lope to it
Then you can start taking it to the track and really have some fun with it.
My Camaro sound clips, clip #2 is stock engine with a loud exhaust. It sounded like every other Camaro and I thought it sucked. But I only had it like that for a week or two before the cam got installed. Clip #3 is with the cam, so with a cam like that it is ok to be loud. Also the cam/other mods dropped me from 28mpg to 15mpg driven normally. So being on ecomodder with a car like that is kind of pointless. But I guess if you want a loud farty/raspy type sound with a stock aluminum engine go for it if that is what you like, everyone has different tastes
But it at best won't change your mileage at worst it will drop your mileage. Unless something is wrong with the exhaust on it now you won't see a gain.