Wow - interesting to hear this result as I was considering doing something with the rear wheel openings on my xB as well as a rear belly panel but now that this comes up it seems maybe some wind tunnel testing of a spinning tire is needed to see what actually happens to air flow around a tire. Guess the best thing would be smooth wheel covers? Even that may turn out to be false and funny if having air pumping outwards by bladed spokes resulted in lower drag.
Unlike the rest, I've been happy with my rear wheel skirts. They added a couple mph to rolling down a nearby hill in neutral starting at 40mph. I replaced the original duck tape with fake leather scrapes and galvanized wire.These do have a space in the back half for air to escape. I didn't plan it that way and now don't plan to fix it.