The only exhaust item that will make a noticeable power increase is down pipes. The mufflers have no meaningful effect on horsepower, maybe 5 to 8BHP at extreme top end. Knock that down by 15% for a dyno number.
2013 BMW 135is 6MT conv JB4/ISO
2011 BMW 335is DCT Coupe JB4/ISO/BMS Flash
2009 Ford Expedition 4X4 EL - 2009 V-Star
2003 Nissan Xterra 4X4 - 1998 Ford Ranger 4X4 Splash
FF Cobra Mk-IV work in progress