Have you spoken lately with the GM customer support personnel? What are they telling you? I would stay engaged with them and make sure you are documenting every visit.
Regarding the exhaust, was this an exhaust that the dealer supplied or that you supplied? Since it was non-GM, I would work with the dealer and let them know that it is making noise and see if they can ensure it is tightened and properly installed.
I own a 2011 2ss convertible and I have had ongoing problems since day 1.
The guys at the dealer are really nice guys and they are trying but I still have the same 2 problems since day 1. Every time it rains my rear carpets are soaked. The water bags have been replaced twice, adjusted twice and carpets replaced twice and I still get water in the car. I have never seen any water entering the car so I assume its a water bag issue. I finally got GM involved against my dealers request and have a complaint #. The car is in the dealer again for its 5th attempt at repair.
Also I had the dealer install a non GM exhaust system when the car was delivered. The exhaust has been banging and rattling around when it's cold since the beginning.