[ASV]

it seems counterintuitive but the large alternator is more efficient than the small alternator. The closer to max capacity an alternator is the more it heats up.
Hot copper is less conductive than cold
if you want your alternator to be more efficient give it some nice cold air
and don't over size the pulley
at higher RPM it easier to produce the necessary power and is better able to cool itself

the only problem I can think of at the moment with the higher RPM is air resistance in the cooling fan