Wasn't it Cobb that was saying they thought the axle back factory exhaust was very restrictive? This definitely proves them right. 16 whp, especially on a Mustang dyno, is huge power increase for just muffler change!
I can't wait to see what happens when the cats go away, a better FMIC is installed and better air intake added! I wouldn't be surprised if the car is gets close to 375 whp on the Mustang