IMO underdrive pulleys (I assume for alternator, wp, ps, etc) are a waste unless you're planning serious racing and probably even under those circumstances. Theory is reduces load on motor by spinning accessories slower but can wreak havoc on the charging system and could lead to cooling problems. Another supposed benefit is it is harder to throw a belt... unless you're planning a race-only car with a battery charger in the pits, I wouldn't recommend but maybe other opinions?

2 cents,

Chris