[ecomodded]

That's a tricky question Cd, i'll five you my theory on it. In 90 percent of windy conditions i would say the side-skirts would aid in reducing the drag co-efficiency. In all but a strong storm like side wind. In that case it would aid you in getting blown off the road.
The forward motion should be your primary concern as the winds will be 30 to 60 mph every time you hit the road.
I will assume that you would be using a front spoiler.