One thing to consider is that even if the FAIRING survives the highway speeds and crosswinds, even a good roof rack might have a problem.
Made the stupid mistake of driving into the garage forgetting the bike was on the roof in a fork-mount Yakima rack (I meant to go riding after work but 30+ MPH gusts made me change my mind). Didn't do any damage to the bike or rack, but ripped the rack right off the top of the car and did some cosmetic (though probably expensive to repair) damage. From this I came to the conclusion that the weak link is the attachment to the car.
This was reinforced when the wife & I went on vacation last summer, and to save interior space (I drive a dinky little Fiesta) I got those racks that mount the wheels to the crossbars. I'd been driving around with this rack all summer, but after 4 hours in the slipstream the additional drag from the wheel mounts slid the rack back about 8", and actually loosened the mounts as they'd been slid to a narrower part of the roof!
Unfortunately I didn't have the keys to the tower mounts with me, and had to pick up some cargo straps to make sure the rack didn't come off. Needless to say, the wheels rode back home inside the car.
Bottom line is that I'd be seriously concerned about the forces on the roof rack (of whatever kind) from that fairing.