Also cant forget that while you can shoot at f11 in limited circumstances your camera is not autofocusing at f11. It is autofucusing at whatever the max apature of the lens is. These would be focusing AT f11. Not easy to be fast or accurate. It would work in perfect conditions. Bright light. Front lit subjects. Very still subjects. And most likely on a tripod. UNLESS as I suggested that maybe the sensors are substantially better at high ISO than we have seen before.
Why no AF @F11 with the R series???
Why only very still objects?
Yes, perfect framing without monopod/beanbag, tripod will be some challenge especially with the 800mm.
To give you an Idea about F11 here a shot with 16 years old technology F10, 500mm x1.4, ISO200, 1/800sec (EOS 1D MII):
Sorry canonrumors had stolen the sun reflection point on the eye :-(
The very first important issue is to be at the right moment at the right location with the right light and the right subject.
Not too bad having equipment with you. You never take around F4.0 just accidentally. (I know, I use F4.0 600mm or 500mm for nearly 3 decades now).