It makes sense, it's just not what happens. You don't get better mpg on a computer-controlled, fuel injected car. Better mpg claims come from carbureted cars. Carburetors' mixtures are affected by air filter restriction. Electronic fuel injection is not. The mixture will be precisely controlled by the computer in accordance with the MAF/MAP sensor and feedback from the O2 sensor.
If you change filters from one with slightly higher restriction to one with slightly lower restriction, you would get an infinitessimal increase in airflow at any given throttle opening. The ECU would then inject that ever so slightly greater amount of fuel to keep the mixture correct, you'd make 1/1000 of a hp more power, and your car would accelerate. So, you'd have to close the throttle ever so slightly to maintain a constant speed. Thus, your air filter may be less restrictive, but your throttle plate is more restrictive by an equal amount and everything is offset; no potential for a mpg increase.
The only exception to this is at WOT. IF one filter is less restrictive than another, then at WOT pumping losses will be lower and you will theoretically get both more hp and more mpg. How ludicrously overloaded is your car that you're spending any amount of time at all at WOT? If you're racing, do you really care about that extra fuel cost compared with all the other factors involved?
