Damn Mark that is impressive.
It turns out I have a "Kenko 1.5x Supertele DG" that has been lying around unused. I also have the 70-300 IS USM (non L) so I may try out this whole TC / autofocus thing. Seems like you are pushing the envelope on the 6D with the MSNR and the TC's - but that shot of your cat Ollie is seriously impressively clean and sharp.
Now, where the hell do I find a cat?