There are very few ways to simultaneously increase MPG, power, etc.

The V-10 straight from the factory is pretty well tuned, you can tweak it around the edges, and get a sustainable single to very low double digit improvement without major rework.

The bottom line is, if you really wanted more power, economy, etc. you needed something built with a bona fide medium or heavy duty diesel on a medium or heavier duty frame.

Even then, you are limited by the reality of only having so much energy per gallon of fuel, and against that, weight, air resistance, etc.