Do I get better gas mileage?

A jackplate increases fuel efficiency. When you channel prop thrust parallel to the surface of the water rather than up or down in comparison, you waste less energy. Additionally, the lower drive unit, prop and skeg all create drag in the water. With a jackplate, you can raise the engine higher in the water column, thereby decreasing drag. Certainly, you may need to lower it further into the water in rough-water conditions or in turns. But calm conditions will allow you to run with the engine higher and burn less fuel.