I saw some samples from the 800 that were not that great. Before diving into the system, maybe rent it to ensure it meets your needs. Remember also you'll most likely be shooting at higher ISO to compensate for f11.
Are you referring to the Osprey birds-in-flight shots at Dpreview?
https://www.dpreview.com/forums/thread/4508663The ospreys don't look at all sharp to me, after clicking on 'the original' images and enlarging. However, the images seem very compressed, or heavily cropped, have very few megapixels, and are also probably OOC jpegs.
Later in the thread, the same photographer shows some comparisons between a 500mm F5.6, used at F6.3 on a Nikon D850, and the 800mm F11 on the 5R. (I think they're herons swallowing a fish). The images from both cameras look much better, but the 800mm F11 shots look better and sharper than the 500mm F5.6 shots.
What's also interesting about the 'Photographic Dynamic Range' charts from BClaff, is that the Canon 5R seems to have 0.87 EV better PDR at ISO 400 than the Nikon Z7. At no point on the ISO scale is the PDR of the Nikon Z7 better than the Canon 5R. I wonder what the DXOMark charts will show.
I imagine that ISO 400 would be a frequently used ISO with an 800mm F11 lens. In bright light, perhaps sufficient for most moving subjects, and certainly for slow moving subjects.