I have a set of Magnaflow street series mufflers for the 13 GT500 for sale. It is the polished version, shipped for $550 or make an offer. Mufflers have around a 1000 miles on them. PM if interested.
I had the black series first, however I had to send them back twice due to the coating peeling off. Supposedly they are getting a new supplier to do the coating shortly. I would not recommend purchasing those just yet.
Looks like they have the problem under control now, good deal! I had one of the first black coated according to Magnaflow and they had to do quite a few returns. I believe they did change the shop which does the coating.