I, too, have been thinking on this subject.
In the days of carburated engines, a dirty air filter could affect mileage to a considerable extent.
However, on the milti-point fuel injection systems, with Oxygen feed back from the exhaust stream, I do not see how the air filter can have much effect. The computer monitors to the exhaust and varies the mixture almost instantaneously to obtain the stociometrically correct ratio. If the intake is restricted ( dirty air filter), the computer will only allow enough fuel to burn the available air.
A dirty air filter may restrict the available power, but I see only a minimal impact on economy.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags