The final cut of the 9th episode of the Tesla Motors Club Podcast, featuring Chad Schwitters, the former president of Plug In America, is now available. You can watch it now on YouTube or listen to it on all major podcast networks.
I had a roof rack on my Model Y and did some unscientific testing. The difference was negligible with a naked roof and the Tesla roof racks. A bit more whistling noise at freeway speeds, but that's all.
With a bicycle rack mount (Yakima Frontloader, which has a large frontal area when folded), my efficiency went from 285 to 295 Wh/mi. Again, this is anecdotal, not any true experimentation.
With a hitch, bike rack, and bicycle (road), and without the roof mounted bike rack, the efficiency was mostly the same (295 Wh/mi).
At the end of the day, the whistling noise more than any efficiency bothered me. So much so that, for my Model 3 (sold the Y), I purchased and installed the Stealth Hitch. It is not that hard to install, although I do have some prior experience installing hitches. The installation on the 3 was only a little more difficult because you have to remove the lights, bumper, and some of the substructure behind the bumper. However, you also do not have to balance the hitch while installing, which is what I had to do with my other vehicles (minivans, SUVs, and economy-cars).