Anywhere between 0.1 hp and 20 hp. Look closer to like 2-5 hp though. There is no way to accurately guess horsepower. Your car may be one of those factory freaks that left with 200+ at the crank, or on the flipside it may be a opposite situation where you left with around 170ish at the crank. Factors like that will change how much you will get.
Short answer though: 1-8 (Being very generous with that 8 though)
Also, running what kind of muffler are you looking at? Some flow better than others, as well as the fact that no cats could actually cost you a few hp.