Assuming a system will drain a battery, if the car is running a Underdrive pulley might be jumping the gun. What kind of amperage is the system running? It is quite possible to safely run a system on an Underdriven alternator, there are too many unknown factors to accurately answer the question.

In terms of performance, the gains are minimal anyway. I agree with JDMjames, read up some more and decide if its really worth the money to begin with.