Tried out F/11 autofocus with the 2X teleconverter today. Insane reach at effectively 1,280mm when you see the crop mode through the viewfinder. Hard to spot your target at that focal length!
Autofocus worked great, sometimes would lose track and end up searching through quite a long distance, but I normally corrected that with the manual focus ring quickly. Rough ISO outside of daylight, but totally usable with the EOS R5/R6. I think this experience today makes me totally understand the F/11 lenses. Even in the shade, it was totally usable for social media and really made for some impressively long reach. For the vast majority of amateur photographers who just post to social media, I very little reason why they wouldn't be happy with the results of a 800mm f/11.
EOS R5, EF-RF Control Ring Adapter, 2X teleconverter III, 100-400mm F/4-5.6L IS II. ISO 5000, F/11, 1/200th. 17 megapixel 1.6X crop mode, effectively 1280mm F/11. Somehow, even at 1280mm, bit of a crop in post as well.