Your comments lead me to believe that I may be expecting too much, especially with the x2 on the 600mm at long distances. I just recently acquired this combination. I was totally satisfied on an earlier shoot using just the 600 at much closer distances. On this most recent shoot I see that using the 600 without the x2 produced the same out-of-focus on the long distance shots.
Combining all the suggestions received I intend to do the following:
Do a test at 100' with and without the x2. Set VR at Off and at Normal --- not Tripod. Keep Wimberley head loose instead of locked down. Try Single Point focus Try Manual and Auto Focus with and without x2 Find out what LLT means and give it a try Try much faster shutter speed (these were at 1/640 sec.) Just don't shoot at such long distances.