In essence the less restriction there is in getting the air in and the exhaust out the better. Cooler air is denser air which burns more fuel producing more power meaning less throttle so less fuel used.
All of this said, the problem is not using that extra power. That Flow Master setup sounds so good, it's hard not getting on it, which of course translates into more fuel burned. To do all these mods and drive like your grandmother is not going to happen.
If you’re worried about fuel mileage leave it alone. The engineers at GM for better or worse worked very hard at getting the best car they could with the restrictions they had. In any case, even if you did drive like your grandmother you would never save enough to pay for the mods.
So what the hell, do it because you like it. Make some noise, impress the girls, give up steak, bologna isn't all that bad when you get use to it.
94