The cat on the GSR is shorter, therefore the LS catback exhaust is too short to fit the GSR. I bought a GSR exhaust and had to get it shortened, but hey I got a brand new Apex N1 for $300, it was worth it. As for performance, they are the same.
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
Team B.O.B.® - Ballaz on a Budget