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.
2014 BMW 435M-Sport 8ZF
2013 BMW 135is 6MT conv JB4/ISO
2009 Ford Expedition 4X4 EL - 2009 V-Star
2003 Nissan Xterra 4X4 - 1998 Ford Ranger 4X4 Splash
FFR Cobra Mk-IV
work in progress