He's using delta shaped VGs,
not airtabs.
I used the Vortekz brand VGs for some time, and found they do a better job of reducing lift than drag. They alter the pressure of the air above the rear, which reduces lift, but the forceful moving of that air creates drag. If anything, on a three box sedan, you can reduce drag by a few points, and almost eliminate rear end lift. Mitsubishi did an in-house study of VGs on the Lancer Evo:
http://www.mitsubishi-motors.com/cor...004/16E_03.pdf
I'm not sure what effect they would have on a rolling shoebox, though.