The formula assumes zero friction. I don't know how much power the v belt saps but I'm sure there is some. Same with the bearings inside the alternator and the little cooling fan and all that jazz. I don't have any numbers for all the little friction loads but I'm sure it adds up. My guess is the alternator costs about 1 hp even if it is only charging 20 or 30 amps. Bottom line I don't think it matters much. If I was building a bracket car to go rounds I'd put an alternator on it. It is only a few pounds of weight and a very small power drain but it cuts down on the monkey motion in the pits. I'd rather not have to hook up the generator every time I pull back into the pits. It is just one more thing to remember to do.
Plus the power required to overcome the inertia of the armature as the engine rpm climbs in each gear. I suspect that in the lower gears, this could be more than the power required for producing electricity.