I got Corsa mufflers, which i must say sound amazing, but since then i have lost 2mpg average (with my wife driving mostly) and seems i feel a little loss in power. I have about 750 miles on car since putting mufflers on and i reset the average MPG guage, which used to show 18.4 and how shows 16.4. I know if i was driving the car mostly then it would go down because i love to hear the exhaust, but my wife drives it during the week while i travel and i rarely drive on weekends because my son loves riding in the Silverado SS (hes 2 and likes to be able to see out the windshield from his car seat).

Same exact model D3 carries, ones made for the STS.

Butt dynos (the feeling in the seat of your pants) can lie, unless you're talking significant changes in rwhp that can't help but be felt. The only real ways to know if your down or up any power is to test the car on the dyno and/or track time. But of course you're going to need reference numbers to compare to, unless once again the car is way down or up.

It's possible that changes to the exhaust that restrict it could affect the car, but just changing to Corsa mufflers, I do not think should as long as they're installed correctly.

I thought the same thing when i had my magnaflow installed in my supercharged truck. What I realized is because of the louder exhaust I didn't get on it as much because of the louder sound. Hard to explain I guess