Hm, this will raise a pretty solid question for me on if I should sell my 100-400 and pick this up or not.
Because of the RF 70-200 being so light and compact, I pretty much only ever grab the 100-400 for birding these days, so it normally has a 1.4x on which leaves it as a 140-560mm F/8 lens. A 200-800 F/9 would be considerably more range and not a whole lot lost on aperture.
If the size is right, and IQ is at least comparable to the EF 100-400mm F/4.5-5.6L IS II with the 1.4x extender attached, I could totally see myself picking this up for my own hobby uses.
Somewhat helping to add to this is that my newspaper provided me with a 200-400mm F/4 for work, which has significantly decreased my need for my 100-400 during the workday.