Old thread dig, but just found article/study related to topic for diesel air filtration:
http://www.fueleconomy.gov/feg/pdfs/..._AirFilter.pdf
Doesn't agree with my experience, they indicate on vehicles tested there was little adverse effect on FE with a dirty filter, main effect was reduced performance.
Here's another Diesel article on air restriction and intakes, not as related but good info:
http://www.genosgarage.com/GenosGara...tems-Part2.pdf