I don't have empirical proof, but I would agree, a hitch mount would seem more efficient and be easier to load/unload. The items at the rear of the car would be mostly in the dead airflow, rather smack in the airflow.
we have been using a Thule roof rack for our tandem (now stolen) and 2 other bikes for 10 years, and just transferred it from our '87 Ciera (donated) to the 2002 Prius purchased to replace the Olds. Wind resistance exists, but does not prove problematic. The rack also enables us to use the hauling ability of the vehicle for items other than bikes. The Thule rack has 'feet' that can be purchased to adapt the system to other cars, and is very secure.
I did get a little paint rub on the Olds, but so far the paint on the roof of the Prius seems to be holding up. I apply a bit of carnauba wax to the roof before installing it, also the clearcoat helps protect somewhat. I do sympathize about the lifting situation however. That tandem got awful heavy when came time to put it back on top. Our new recumbents are not much better. The only problem with recumbents is the wheel length is longer that the car (tandem). Happy Trails!.