As far as high xB mileage I have gotten over 50 a couple of times usually if I start at a high elevation of about 300 feet and come down to sea level and it also helps to start off with a warm engine. The only aero mods are the racing disks wheel covers on the stock steelies. The CD is .35 so it is not too bad it just has a lot of frontal area. Nice front belly pan Improbcat - if you were running Synlube you wouldn't have to worry about changing the oil!
Formula Two - it only takes one brisk acceleration up to 45 mph when the engine is cold to really kill the mileage for a 20 mile trip so trying to get up into the 50's is really tough. I managed 44mpg on a trip in the 60-65 with a blast or two in the 80's but that's with the racing disks and all the Synlube in my engine and tranny. Usually I average 43 mpg on long highway trips.
I wouldn't keep the passenger side grill blocked in the summer as I think some of that air bypasses the radiator and cools the A/C compressor and Alternator and the belts.
AirTabs are supposed to work pretty good - still can't justify that cost yet.