I cannot speak to the hp savings though I found out an underdrive pulley caused my wipers to move slow while sitting at stoplights with the headlights on. Also, the coolant warmed up a little at stoplights on hot days. Since I have a truck and not racing, I replaced the underdrive crank pulley with a larger pulley. Now the coolant stays the same or cools slightly idling and the wipers maintain speed with the lights on while idling.
If I remember correctly, my crank pulley and water pump pulley are now 1:1.