I would be surprised if a muffler added any power. Maybe 1 or 2 hp if you must be optimistic. Hopefully it doesn't cause any loss of power! I'm not sure if my Eisenmann Race made any difference plus or minus, but sometimes I feel that my car does not accelerate as fast ever since I installed the AFE filter.
2012 Audi R8 V10, 6MT, Suzuka Gray, Capristo X-Pipe, Klassen M52R, GMG springs, GT tails, GT wing, Cosmetic mods
2015 BMW X5 35d, Mineral White
2016 MB C63S, Steel Grey
Previous: 11 e82 1M, Several 911's & M3's