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.
Like Andy mentioned, the loss to electrical load alone is pretty small. Once the battery is fully charged the alternator need to supply the amperage to run whatever electric items the car is using, maybe 20-30 Amps unless you have a killer fuel pump or cooling fan. Note that the electrical load loss in not related to RPM.
The other alternator losses like the cooling fan, (drag force) increase with RPM.
The mass of rotating parts and friction will create a load when changing speed (acceleration) or F=Ma
As I understand it, In theory friction force does not increase with velocity, but I think real friction forces (viscous and unbalance of rotating parts) increases with both Velocity and Acceleration.