The question arises... "Do TC's create vibrations due to mechanical fittings?"

These images all shot at 1/10s F/8 with the Feisol 3442 (flat plate, all 4 legs fully extended), Markins M10. The first image, for reference, is the 300/4 AFS with Kirk Collar. The next 3 sets 70-200 F/2.8 AFS/VR with TC14 at 280mm, 200mm, and 150mm.

The 200mm image is a bit soft, which I think confuses the issue. May have been focus or maybe the lens and TC don't like that focal length. The 150mm image, though, is sharp with all techniques. From this I concluded that the TC likely does not, in itself, create excessive vibrations.

The 300/4 images show a bit less vibration, as expected with the superior design of the Kirk 2 point suspension collar.