A rather quick, non-scientific test here. I put the 500F4LII on a tripod, added the 2xiii and took shots with the 5Diii and the new 7Dii.
I've cropped the images to be as near to identical as I could without spending too much time on it. The images have had exactly the same level of (slight) processing. Needless to say, the 5Diii images have been cropped to a greater extent than those of the 7Dii in order to acheive the same effective shot.
Left side is the 5Diii / Right is 7Dii
(This area of the moon is known as the 'Sea of Crisis'...!)
To me, the difference is clear, and you can see several small craters in the second image which are not visible at all on the first image.
Not drawing any difinitive conclusions from this and will have to wait until daytime to test the new camera on some 'birdies'. However, based purely on this quick test, I have to say that my desire to have an effective 1600mm tool (!!!)

would appear to have been met.
Doubt if it will help me take better pictures though......