In the older cars it was 15 hp for every .1 but yeah these new cars don't have as much drag so maybe 10 hp it is now.
I still don't believe it either, with those Stang's-and many other cars-its hard to hook up from a dead stop because the back end is so light. 20 mph to 70 mph gives a better feel for checking if the motor has more power since wheel spin is less.
Back to the water pumps, if your are running a electric fan-which it some places you can-I haven't even used mine here in Michigan since Nov.--the water pumps seem to last forever. And when they do start to go, they either start leaking a little or start making a funny noise, they do not just die out on you like a electric pump might.
That 5-10hp might well be right and thats at high rpms, IDK, you might notice a diff. you might not, guess a electric pump can't hurt unless it breaks down on you, then its tow truck time.