It's a plastic lens cover, hit it with a 1000 grit wet dry sand paper if bad and then buff it out with a plastic rubbing compound. it will look good as new.
I had the same problem, I removed the resonator and routed the exhaust out the side last weekend, not straight back. It helped considerably but there is a hint when I really get on it. Nothing when driving around normally. btw slight tone change when removing the resonator.
