it means you can run 80 amps of draw, rather than the factory 40 or 50 etc
so you can run more electrics, like thermos, lots of lights, electric winch etc etc, should maintain spark strength better too.
dunno if it charges faster, doubt it...
underdriving it means it spins slower. so less charge down low, not really a good thing. the pulley is pretty small already, i would only underdrive it if it was a race car, and was definately going to spend the majority of its life high in the rpm.
obviously the benefit is, the slower you spin it, the less drag it creates.
the difference it would make, i don't know.
but having an alt running to not running is a big difference in load, ie, it was spinning, but not charging, once i connected it to signal to charge, the idle dropped along way, so i don't think it would make a huge difference, as alot of the load is created just because its charging, dunno how much difference speed makes...