Tested my old 600mm A* 645 lens today. The picture below of the pigeon is with the 600mm lens and TC1.4 at f/6.7 ISO3200 1/1000 and cropped 40%. I'm quite happy with the result. I also had to unpack my heavy duty tripod because my normal hiking tripod could not keep it still with the TC 1.4 which equates to 840mm focal length. The aperture of 6.7 is what is set on the dial, but is actually f/11. Unfortunately the 645 TC does not translate the aperture setting, but merely passes it through to the camera. I found closing the aperture more reduces the sharpness, and at f/5.6 (actual f/9) the DoF is too shallow to be used practically.
After a couple of test shots I pushed the mounting plate forward on the head so that the whole rig is unbalanced and forward leaning. This added some tension to the setup and reduced the vibrations a lot. This increased sharpness a lot, so much so that on a 200% crop you can read the label on the signal booster and make out that it was made in Japan.