[Klaus]

1200mm f/8
https://www.canon.co.uk/lenses/rf-1200mm...fications/

800mm f/5.6
https://www.canon.co.uk/lenses/rf-800mm-...fications/

I'm not overly impressed when looking at the optical design, to be honest.

The 1200mm f/8 is basically the 600mm f/4 with 2x converter
The 800mm f/5.6 is the 400mm f/2.8 with 2x converter

There's surely a little gain in quality due to the integration but there's probably not too much in it.