are underdrive pulleys a good idea? Do you really want to divert the HP away from other components of the car?

 

You are not really diverting HP away from anything. All your doing is changing the powerload on the accessories in order to cut down on extra power being used. Less power being taken up in the accessories means more power for everything else, hence HP gains. Thats why they're Underdrive Pullies, they UNDERDRIVE your accessories (non essential performance parts) for more power gains.