Thanks for that detailed explanation. It'll come in handy if/when I upgrade to a second body.
I might add that lots is possible with the 6D. I use center point spot and that is virtually useless with BIF becasue it's too dificult to get the initial lock (talking 600mm here). By using all the points the focus will often not be where you'd prefer on the head/eye. The frames per second is a little slow but that's not as big a deal.
If a 6D was to be my only camera I wouldn't buy it without accepting the limitations that many have pointed out. However, I've had a very rewarding summer/fall season with that camera and acquired many beautiful shots, just not many BIF. I've posted more than enough to confirm that it's not a bad camera by any stretch.