Typically if you were to install a power-adder, it would hurt your mpg's. People like hearing the sound of the intake or exhaust so they end up using more gas. I know when i put a Volant and Flowmaster 50 series in my titan, i lost atleast 3-4 mpgs and my right foot became heavier!
To improve mpg's, start by making sure your tire pressure is good. Then work your way through the process: check your spark plugs, your filter, efi, etc.
The best way to improve your mpg's is to improve your driving habits. The lighter your right foot, the better your gas mileage is gonna be. Good luck. :cheers: