That would line up pretty well with the Nikon, but will Canon throw in a custom extender
How did the prices change when the 300, 400 and 600 Mark IIs were launched? How much more were they when they came out compared to the Mark Is?
The MkII versions of the 300, 400, 500 and 600mm supertele lenses launched at price increases of 40-50% over their predecessors. An equivalent increase would put the 800/5.6 II at ~$19,000, give or take a few hundred.
Again, it would have to be an amazing lens to justify $19k, with or without an extender.
800mm is pretty long, so the 600 f4L IS II gives you better close(r) range capability and you have excellent performance with th 1.4xIII. I had the same issue with the 400 f2.8L IS II, before buying the 600. With the 1.4x and 2x extenders it gives you very good focal length alternatives. It works well at 640mm, but the IQ at 800 is significantly behind the 840 you get with the 600, with which you can go to 1200. With the 400 you get f2.8, which is a very usable feature, wheras with the 800 you get same f-stops as the 600 with extenders.
At 19k you get a 600 with a 1DX hooked on ... So, to justify that price tag, the 800 would have to be truly exceptional.