Increasing the oxygen in the air or decreasing the fuel in the air is the best way to get better performance and increase fuel mileage, but you need to increase fuel to air when under acceleration or as load increases. As for increased power, there is no substitute for cubic inches other than forced induction and both require more air thus more fuel. Tuners only work if you have a working knowledge of the changes you are making and can see the the results of the changes you make on a dyno chart.