I can also testify that the 600 mk II with 1.4x is sharper than the bare 800mm. It is not by a lot, but still noticeable. The bonus is that the 600mm mk II can take the 2x very well also. I've been able to get great results with that combo.
However, for those that want to get past 500mm, I think a used 800mm is now an excellent value. I would only assume that a new 800mm II would be priced around 15k or more (Nikon version is what 18k?). I am still not sure what a 800mm f/5.6 II + 1.4x III would give as an advantage over the 600mm II + 2x III.
600mm II + 2x III on 1Dx:
another at 1200mm: