Be sure its the Pro 300 DGX version. There are some who say that only recent production of these will AF at f/8.Loren
I do bird photography and use spot autofocus with the centre spot. I have read that the outer zones are more sensitive but the centre focused well.
I'll try spot; the regular center point just hunts with the 100-400+1.4.
I did use the center poing, but not spot. My Canon 10D doesn't even have spot, but it worked very well as did my 5D MK III.