Bryan, I'm sure Alan F could give the best comment since he has both camera and lens. With the 1DX2 400 is amazing, 560 is great and 800 is very good. I never sense it's hunting unless of course there is simply nothing it's capable of focusing on. However, only the center region of points are cross at 800 so if you use the others it's not as good (but they do work fairly well).
I seriously considered the 500 but cost and weight were not attractive to me. I knew I could lug 300 X2 all day and so the 400 being the same weight and slightly smaller, was my choice. I can't say I wouldn't like 500 but 600 would be a pain for my mobile preference. I rarely use a tripod.
Jack