I rented the 800 and took it to Svalbard..shared the expense with another photographer. We both had the 500 that we took with us as well. The 800 was very good without an extender, even the version III extender made the image soft. After the new 600 came out I sold my 500 and purchased the 600 II....when I combine it with the 1.4 III extender the quality of my pictures are SHARPER than the 800 was without an extender....so I now get 840 reach, with a lighter lense and a superior quality picture...and if I don't need the extender I have a little more flexibility with the 600 than the 800 provided....combined with my 1Dx and my recently purchased 200-400 I think that I have all of the bases covered, telephoto wise.....I am keeping both of these lenses.....you never know when you need the 840 reach when you are shooting wildlife....but can't handhold the 600, not and still get quality shots all of the time.....
I have the Canon 1.4 Mk2 extender and find that it works reasonably well with my 800 F5.6 though there is some IQ loss. The Canon 800 is not the best lens for use with extenders.
When I was buying I picked up my, used but near mint, Canon 800 F5.6 for abut 45% of the cost of a new Canon 600 F4 Mk2. I would prefer the 600 Mk2 but the price is simply out of the question!
Pity - I quite fancy one.