I have 2 old ENEL-4 batteries that I got with my D2h years ago and they are both '4's now. I use Eneloop 2000ma AA as backup. I don't use the AA's a lot but when I have they seem to work fine and I think I get the 8fps rate.
The only thing I don't like about the AA pack is that the battery meter is not accurate and I think they tend to die fast without much warning. That is typical of any NiMH cells, which are very difficult to meter remaining life. I don't have enough experience to suggest a realistic useful life in terms of frame count.
If I were starting over I would try a set of Eneloops and the battery holder. A lot cheaper than the ENEL-4 and charger, and AA cells are usually useful for other tings like speedlights, especially Eneloops.
For Af settings, I would add that I use 9 pts whenever possible for BIF, and would only use 21 if I can;t do it with 9. Lesser is better but you need more accuracy in your panning.
You might try AF-C in focus priority mode just to get some feedback on how well you are tracking. In release priority mode the camera does not care and therefore you get no feedback.
I usually shoot BIF in AF-C focus priority but I give up a lot of shots that I might find useful. I suspect that the number of in focus shots also increases since the camera has time to time the shots in sync with when it thinks it is in focus. But I have no way to know how to test that, given the reality of this shooting.
It is a direct trade-off of quantity verses quality without hard numbers to help with a decision. I don't recommend either- I just suggest considering the implications and trying both methods.
For planned BIF shots in varying light I usually use Auto-ISO, with an appropriate minimum shutter speed for the lens (along the lines of Ned's suggestions). I will shoot aperture priority at the widest aperture I am comfortable with, considering the lens.
When the light is low the ISO goes up to maintain my minimum shutter speed. When the light increases I get bonus shutter speed beyond what I would get in Shutter Priority. I can tinker with the ISO setting according to the light and how much shutter speed I might want. The camera is then doing exactly what I would do in manual exposure mode if I had time to think about it and change settings.