Im really interested in what you decide to do as I am waiting for the Sigma 150-600 to see how it compares to the Canon 200-400. Obviously its not going to be the same but will the Canon be worth $10000 extra? I dont mind paying if it is but for a lens that wont be my primary Im kinda thinking about the Sigma.. .hoping there is some reviews coming out asap!
As I am gaining experience with my photography and getting older I am realizing I do not need the most heavy or expensive equipment to get the photo. I am sure the focus of the Sigma will be as quick as the Canon in good light. The difference will be in low light only. So if I spend 40 days in 'bush' every year then I will have 80 mornings/evenings. Out of that I will witness something to photograph 20 times in tough light. Out of the 20 I will miss just few shots. I think I am ok with that. (Actually I do not think I will miss ANY.)
F4 is bit too wide for me. I find that many times the entire animal is not in focus. So 5.6 is fine and preferable. And the real world depth of field difference between f4 and 5.6 is negligible.
The main factor is resale value. Sigma may not hold up to its value after 5 years but Canon will.
I think I will sell the Canon and get the Sigma. Mainly because I prefer to work without the extra step of 1.4x switch. I will prefer the continues zoom option without having to wait for the lens to normalize.
Or perhaps I am building all this up as I want to save the 9k. haahaha