I have the 355 rear end. I can't compare it to yours, so I am not sure how much of a difference it will make performance wise. But I was getting 24 mpg originally. After getting my Pro Cal tuner and the Flowmaster cat back pro series system, I get 28 on the highway. So I don't think mpg will change much at all.
I just started a new thread as a review for the cat back system. You might want to read it if you were considering an exhaust upgrade. Drone is bad. Real bad.
2011 black C/S. Stickers are gone, 5.0 badge in place. Flowmaster cat back Pro Series/Hushpower exhaust that comes with the worst drone in history! FRPP tuner. Drop in K&N filter. Auto Tans
I sit in traffic a lot. HID/security package. 3.55 rear end.