I shot track this weekend at the Jesse Owens Classic for AU, Denison, Eastern Kentucky, and Michigan State. I didn't back down and dove right in with the 400mm v1 anyways, and made some changes in my autofocus settings and technique. I will have those results tonight.
I'm not sure if the 400 focuses slow, but it definitely does "slower" vs. the 300. I'm serious, as I've noticed this even before you asked. I did shoot some of the runners the SAME distance as with my 300 just to see, and then filled the frame, and had the same trouble with both. I made some changes, however, which I'll explain.
I went back to AF 101 and just used a single point, one vertical position above center, and two vertical positions if needed. I then locked onto the runners face, and did so on the right edge of the face, and sort of let the face come into the AF point just a slight bit more before firing the shutter. The hit rate went up. I also changed to tracking case #2, instead of using 2, 5, and 6. Not sure if this makes sense, but again, the hit rate is signficantly higher than last week.
You can do EC with auto ISO with the 1DX, however, you have to shoot in Tv or Av mode. You can fool it into doing the shutter and aperture you want, by setting upper and lower limits on each. Then just turn the dial to + or - whatever you want.
An example. The 70-200 f/2.8L II IS lens widest is f/2.8. Set the 1DX to Tv mode, at 1/500s, and EC to +2/3. Then sit max aperture to f/1.0, and min to f/2.8. ISO is auto. You force the aperture to be at f/2.8 at all times. Of course if you want a narrower aperture, it can get tricky. The short answer is no, and the long asterisk answer is yes, sort of.
Thanks for the help everyone. I feel as though I gained enough to start fixing my problems. Again, thanks.