It is one of the most effective ways to pick up some power lost to things like stock alternators that have a high demand and cause drag, and power streering, and AC. I put in an under drive pulley set from Mac, and the "butt dyno" did feel the difference. On a real dyno, it does show a significant increase in torque, which would be lost wit overdrive pulleys and higher demand for electrical power, which causes drag from the alternator.
Some of the ricers with high powered sound systems, mount two alterntors on there motors, one dedicated just for the sound. Now, that causes a lot of drag.