A couple of things could cause yer mileage problem:
1) Oxygenated fuels.... these have a tendency to lower mileage,
which you'll see when the winter switch comes. Does your station
switch? I know some in the DC area do, though I don't think its
mandated anymore....could be wrong (wouldn't be the first time).
2) Didn't you say that you have a lower temp switch/thermostat
(or something to that effect)? I don't know what the ideal
operating temp is for your car, but mine doesn't seem happy until
the temp guage is 1/3 to 1/2 way up.
Though my GT doesn't have an air damn/front spoiler, I have had a
few BMWs that did have the Alpina spoilers mounted on them, and I
think the mileage actually increased (though not drastically, but
when you live in Germany and gas in roughly $4 a gallon, little
improvements in mileage you notice.....). Same thing happened to
my Lancias as well. These were pretty unnoticeable front
spoilers, not the Darth Vader looking things you see on Maximas
and Escorts here in DC.