I have several friends who have multiple Canon super-telephotos, but I do not know anyone else with both a 600v2 and an 800. I am very curious about something, and you may be one of the only people that can answer my question with an answer that comes from lots and lots of personal experience:
How do you find the image quality to be, comparing images taken with the 600 v2 and the 1.4 tele-extender, to images taken with the 800mm with no tele-extender?
Tom I would give the edge to the 800mm which is my mainly used lens, but they are so close. I honestly do not like taking extenders on and off that much. It just takes to much time taking them on and off and in dusty areas it could be an issue too. If Canon built a 600mm with the built in extender like the 200-400mm then I would love that over the 800mm. I have both lenses because both my wife and I shoot. She will use the 600mm when she can be setup on a tripod. I handhold my 800 a lot too and the 800 is a little smaller then the 600mm in size which I actually like when handholding and packing. I just picked up the 400DO II for mainly a 800mm light traveling lens. I will basically leave a 2x on it most of the time for what I want it for. So far that is an amazing piece of glass with the 2x on it. I have not had much testing so far but boy so far it is very impressive.