I cannot give a definitve answer. For a vertical cardinal, I can probably fill 75% of the frame (the largest that I prefer to go) at about 25+ or so feet with the 800 alone and the Mark III (1.3 crop factor).
When you go from one telephoto to another with a different focal length it takes a while to get the distances down right, i.e., when to stop when you are approaching a subject. Remember: the size of the subject in the frame is a factor of the square of the focal length. So going from a 500 (5 squared = 25) to an 800 (8 squared is 64), the bird will be 2.56 times larger in the frame (64/25). Often times I walk up to a bird with the 800 intending to create an image of the whole bird and find that I have to back way up... Hope that that helps.