You know, Im lookin at my bike rack, w/fairing, and I would almost venture to say that the fairing doesnt help at all. It just increases the overall frontal area of the car. The tubes arent very big, I think a little "V" behind them to make them look like a teardrop from the side would be better. I ordered a scanguage so as soon as I get it Ill run some tests:
*bike rack w/ fairing and no bike
*bike rack w/o fairing and no bike
*no bike rack
And if the tests go by fairly quick I'll throw a bike up there as well.
Yakima makes an aero add-on for cross bars. They're about 6 to 8 inches wide, so you may need to get several in order to notice a difference. Also note in the description that the Windjammer and fairings are intended to reduce wind noise, not drag. Although to me, noise = turbulence, which usually means more drag.