Hmmm... I may be wrong (has happened before
) but my understanding is that the battery gets drained a bit when you start the car (to power the fuel pump, ignition and starter motor). As soon as the engine is up to speed the alternator will start recharging the battery (time to fully recharge depends on revs and how much it took to start the car). Once the alternator is spinning, though, it supplies all the current for the car (as well as charging the battery) up to its maximum output. The latter usually depends on revs. At tickover it should cope with a fuel pump and the ignition, but may not be up to headlights and a rad fan running. As soon as you start driving, though, the alternator should supply all the current demands and still keep the battery fully charged. In this situation you can think of the battery as a sort of "back up" which will take over supplying current if the alternator fails or isn't big enough for the demand.
Anyway, the more things you have swithed on, and the bigger your equipment
the more current the alternator has to put out, so the more power it takes from the engine. The figure of 5hp will only apply to one situation, but doesn't sound unreasonable for peak demand on an alternator. Beware, though, that this peak may only be what is actually being used on a dark rainy day with all your lights on, the windscreen wipers running, the demist fan on max, and your engine is overheating so the rad fan cuts in!! On a cool sunny day your alternator will be taking much less. Soooo.... what I'm getting at is the high revs cut-out theory for the alternator is sound, but might not give you nearly the power improvement you were expecting. It also will not give you any extra power at lower revs, which might be important to your acceleration out of corners rather than your top speed.
I don't know how well the battery would cope with the constant charge/discharge cycle, but I would have thought a proper race battery would handle it better than a wet battery. A possible improvement on the whole idea would be to look for a system which is linked to throttle position and cuts the alternator when you floor it, but lets it draw hp when you are feathering or off the throttle... then there should be no down side at all