FRG, are you and H2Finally friends?
More air in, more air out. With a larger, properly designed exhaust the engine does not have to work as hard to expell the gases through the muffler and exhaust pipes. It would then make sense you are freeing up horsepower.
Sort of like adding an under drive pulley. The engine does not work as hard to turn all the pulleys, thus freeing up HP. (Don't call me out on that sentence, I know it turns them slower.)
Black Lingenfelter Lux