Lower setting on the a/c blower makes the a/c compressor trip (cut out) more frequently thus the better mileage. If it is required to prove this mathematically, it can be done. The equation is very simple.
As far the problem of low air flow though the a/c vents is concerned, you may have a clogged a/c evaporator (located inside the car). By clogged I mean that there is muck on the fins, not internally where the a/c gas flows. Assuming the evaporator is dirty, cleaning it externally will improve the air flow plus improve the cooling inside the car.