I'm just as confused John but that's normal for me! Don't you mean feet not metres in your reply but that increases the MFD of 6m with the 21mm extender. It also means, and is what I found frustrating, the narrow band where the bird has to land to be able to focus dictates that anything outside and beyond is no good.However, if I read it correctly you are saying if they are that far away they are not worth photographing as they are too small in the frame?
No I do mean meters the 800 focuses down to roughly 19/20 feet (6m). Normally it is irrelevant but at a couple of hides it is just a touch too long.
So for a small reduction in minimum focus distance I loose a lot of the maximum focus range but for smaller species this does not matter as they will be too small in the frame at the furthest range I can focus with the 13 or 21mm extension tubes. I hope that makes sense!
I don't often get larger species coming in very close but occasionally.........