I recently installed a Gibson cat-back exhaust and a K&N 77 air intake on my 2011 Silverado witha 4.8. Haven't noticed much in power gains and it's too early to tell if the mileage is better. What I do notice is a slight delay in RPM's when I hit the gas and then the RPM's stay raised for a while before they slowly come back down (I noticed this when reving the motor in neutral and when driving slowly). I don't know if this is normal or if it was like this with the stock exhaust and air intake. Is there some kind of a "tune" I have to do so the new system "works" better?