I did a set up for a customer on a GTS ... I used side exhaust sills and welded on a flange at the side port with a aluminum cover to block off when he is not at the track. He can use side exhaust or cap it off and it will be rear exit for normal every day driving. I tested it on a dyno and the car made 8 rwhp and 11 rwtq more with the caps off...I never knew how restrictive the rear muffler really was. It is odd to see with both, but the customer wanted a quiet car for daily driving and a little edge at the track. I want side exhaust for my car, but I don't want my rear fascia left open. Autoform sells a filler panel, but I am afraid you would be able to see a seam after it was blended in...it is tough to patch something that size. I am not sure if a RT 10 rear fascia will fit on the back of a GTS...if it would I would change to side exhaust in a second. If you want, I will try to get a pic from Tom and get it to you. Hope that helps.
Later,
Joe