Darn you Californians and your good testing weather.
I've attempted to test rear skirts 3 times on 3 different vehicles:
details here
In all cases, the tests were less than perfect, despite 2 of the 3 being A-B-A style (I concluded based on the 2 sets of A data that the drivetrain temps hadn't stablized).
Results were between 0.6% MPG improvement to 2.7%, with the largest impact seen on the Metro (which I think is due to the design & placement of the wheel arch).