I took these images this morning with the R5 + EF 100-400mm II + 2.0 TC III + EF-RF adapter.
It was heavily overcast so I had to use higher ISO and lower shutter speeds than I would ordinarily like but I think the combination is usable for perched birds.
The AF is only slightly slower, the eye tracking works perfectly and all focus points are available.
One note .... when zoomed all the way to 400mm you end up with an 800mm f/11 lens .... so I don't know how this combo would compare to the RF 800 f/11.
The RF 800 f/11 is a bit more expensive than the 2X TC III especially if you can find a good used one. Also would be less versatile.
The 100-400mm II without the 2XTC is quite a bit sharper but I have no doubt that good images can be made with this combination, especially in better light than I had this morning.
I do not have a version II 2X TC to compare.
800mm f/11 1/250 iso 4000
541mm. f/10. 1/125. iso 3200
800mm. f/11. 1/250. iso 4000
556mm. f/10. 1/100. iso 3200
482mm. f/10. 1/125. iso 3200