Based on a thread in another forum, I've discovered some information that makes me rethink my AF choices.
The D800/800E uses the Multi-Cam 3500 FX focus module. With that module comes AF at f/8. But there is a big difference in AF across sensors. Keep in mind all this is based on lens or lens+teleconverter combinations wide open.
With a lens that is at f/5.6 wide open, the central 15 sensors are cross type sensors and all sensors will AF. The outer 18 sensors on each side operate as regular line sensors.
With a lens that is >f/5.6 but < f/8, the center sensor and the adjoining 8 sensors are cross sensors, and the central 3 sensors to the left and right of this group are line sensors. Others sensors will not reliably AF.
With a lens that is f/8, only the center sensor is a cross type sensor and the line sensors are the four sensors to the left and right, plus the one sensor above and below the center sensor.
This has implications on the choice of sensors when you are using a lens with a variable aperture or a teleconverter.
If you are expecting to track a subject with AFC, you have some increased likelihood of losing focus as the subject moves through the frame. But there is nothing you can do.
The center sensor is the one that performs best and is a cross type sensor while using other sensors means you are more likely to miss AF, but it will depend on your lens. In any event the sensors outside of the center sensor are less reliable.
All of the diagrams for 11 point AF on current cameras show there are no cross type sensors - and the diagrams are next to diagrams of the 51 point AF which have highlighted cross sensors. I think this means that there are no cross sensors with 11 point AF but I don't have confirmation. I have previously chosen 11 points for speed in moving the cursor across the frame, and this will cause me to rethink that decision and choose 51 points.