Gains?
Yes, some. Will the ENGINE make more power? NO - more will be available to drive the wheels vs. the accessories however.
There *can* be mileage improvements, but they are typically less than 1 MPG, so you will never recover the cost.
I wuld NOT get that style of UD pulley, called a "piggy back", but rahter the style that actually replaces the dampener/crank pulley with a NEW and smaller one.
Contact Chris at
www.adtr.net for things like this, he is a GREAT guy!