Lets see.
The alternator will pull the same energy at any speed, the voltage regulator will provide more power to the field windings to make up for the slower speed, so no real gain there, and at idle it may not be able to keep up with the load.
The water pump will turn a bit slower, but pumping water doesn't use much energy.
The Power steering pump will turn slower, again not much power going there as it is called into duty only when turning.
The airconditioner, if you are worried about beating the next car, a full dip into the throttle will disconnect the ac clutch anyway, again no gain there. Or you could just turn off the AC when 'racing'
I just don't believe that there is a recognizable gain in available power with an underdrive pulley. It is true that you will be able to spin up the engine a very little bit quicker but I'd defy anyone to be able to measure it with the seat of their pants. But a neat shiney chrome one is cool looking, that is if the passenger wheel is off and the inner pulley cover is removed.