I'm looking at high performane pulleys that they claim add power and better gas mileage. True or false?

You can get underdrive pulleys from an assortment of mfg's. They slow down all your accessories to reduce engine load. Usually good for a negligable improvement of output. However it reduces the output of your accessories (alternator/pwr steering/AC). It wont seriously hinder their operation but its is noticable.