Underdrive pullies won't give you a magical 10hp. They take some effort off of the engine driving the accessories. It could have negative effects on the alternator though. I wouldn't really waste your money on it unless your car is supercharged.

edit: My friend says that you have to be careful with the electrical accessories because with your alternator not producing as much power, you can drain the battery quickly. This tends to be more of a problem on Japanese cars where the electrical systems are marginal as far as power goes.