I am in the fortunate position to be planning on buying a long reach telephoto lens.
I would be using this lens for nature photography ; birds & mammals reptiles etc.
The majority of these are elusive and small. But they are also fast moving/twitchy.

I have been using the 100-400 and have added a x1.4 extender v3 when I felt I needed the extra reach but the image quality is not so usable so I feel I need a longer reach telephoto.

With the new 600mm it has 2 fluorite elements and is f/4 so the extra speed would be very nice. Adding a 1.4x would get me to 840mm (though again image quality will degrade).

So the 800mm has the reach. This has only one fluorite element but I know the image quality is still superb. I use the 1Ds mark iii so I understand that this will autofocus down to f/8 which means with the 800 I could even use up to the x2 extender (though image quality may be unusable?). Not sure if a version II of the 800mm is coming?

So I have flexibilty and speed with the 600 or permanent reach but slower on the 800mm.

Any helpful advice gratefully received