I just drive'em

If I understand this correctly, underdrive pulleys slow down accesories, therfore reducing altenator output. By how much? Then zxtuner has an altenator overdrive pulley increasing output… cool. Is water pump efficiency decreased? Please help.

You are correct. The underdrive pulley decreases rotational speed of the accesories. This is why there is an Alternator pulley that can also be purchased incase your car draws more current then what the alternator puts out when the underdrive pulley is in.

Its just a theory but a udp could prevent water pump cavitation at high rpm. If im correct this would be perfect for a high revving engine that sees the high end of the tach on a regular basis. This would increase cooling efficiency for such cars and offer greater longevity.