25% HP loss is a very high over-estimation. It varies by car. By that math, the 305hp 3.7 would put less than 230hp to the wheels, when in stock form they put down anywhere from 260 to 270rwhp.
Did a couple of searches and i think i figured it out! Whatever a mustang makes on rwhp, it makes 15% more of that power on motor hp.
Eg. The shelby gt500, ford say theyr 662hp. But multiply that by 0.85 which is 562rwhp. And thats what it should make on the dyno i guess