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, Larini Clubsport exhaust, GMG springs, Satin Black badges
2012 Audi A6 Prestige, Moonlight Blue
2009 Lexus ES350, Sparkling Graphite
Previous: 11 e82 1M, Several 911's & M3's