[davidmanze]

canon RF 800mmf5.6 is canon 400mm f2.8 plus 2x teleconverter  plus RF  mount
rf 1200is just  600f4 plus teleconverter and RF  mount.
Dunno why someone would pay a premium price for any of them when he can get 400mmf2.8 plus the teleconverter plus mount adapter for a cheaper price and have  a more versatile equipment.
It's not the first time Canon does this, in the past they made FD 1200mmf5.6 then converted the same lenses to EF mount they even included teleconverters for some
https://en.wikipedia.org/wiki/Canon_EF_1200mm_lens

Have you anything factual to back up these claims?   After reading a review of the 400mm F2.8 RF using a the Canon 2X TC and I quote:


"the image sharpness hit with the 2X extender is stronger than with the 1.4X TC (already softer in the center) and the 800mm F5.6 results are a bit soft (though the circle periphery seems to take less of a hit"

  that leads me to think 400mm RF F2.8 and the1200mm F5.6 are two different lenses, as the sharpness is clearly not of the same caliber .......

  let's not assume things from the practices of the past ....