I always look at a decrease in engine efficiency/power as a decrease in MPG. It simply means that same car that would otherwise go from 0-XXmph in X seconds, now takes longer, making you accelerate longer, burning more fuel. Basically with the loss of power you have to push down more on the throttle to get the same results.
Now of course if normal maintenance is done, even a "dirty" air filter shouldn't be bad enough to cause a choke of power of that magnitude.