For what it is worth, I use my 80mm, F7 (works out to 560mm) ED refractor on my Nikons not uncommonly. I am quite sure it is not as good as the 800mm Nikkor, and my telescope has a fixed aperture (no internal diaphram like a camera lens has), but you know something - for the price of the 800mm, I could buy 18 of these ED refractors.
Again, your mileage can and will vary, but once you get 300mm and beyond in any brand of camera lens, IMO, you are getting into a real specialty market. I have a beautiful 300mm Nikkor, but I seldom use it outside of birding, which is not all that often. If you are covering the Olympics or the Super Bowl, I can see where that 800mm lens (or something similar) will come in handy, but how many of us actually are doing that kind of photography on a regular basis?