Use your 100-400 and crop. You don't need anything better for these purposes.
I disagree. Even on a video monitor, I think a long lens can produce far superior photos than cropping a 400. Of course, it depends on what you are photographing. I like taking photos of birds and critters. I used my 400mm with a 1.4 extender on my 7D crop body and tried to take a photo of a bald eagle about 100+ yards away. That was the equivalent of 896mm and I still had major cropping. It came out "ok" but I know if I had an 800mm (for instance), it would have been much nicer, without the severe cropping. And that would be if I was still talking about my 7D. With the 5D 400mm wouldn't even come close to what a longer lens would have produced. It isn't always possible to get close.
BTW, I did pick up an EF 2EX II last night. I'll give that a test with my 100-400 and see how it does. But that puts it at f11, on my 5D3
I also checked into renting an 800mm, just to see what it's like. They wanted $8000 for a deposit, just to rent one.