After having the lens and 2x converter now for several months I've come to the conclusion that I need the lens rail. I've shot this combination both on tripod and window/ground mount and in each case while I get focus confirmation I still must hold either the lens or the camera body which adds to a certain amount of vibration. In looking at the files I can see that the image is ever so "off". The good news is that I can clean up the file using Photoshop Shake Reduction.
In the end, I've decided a call to Really Right Stuff is need tomorrow morning.