[photonius]

If you buy an RF 800/600mm f/11 size isn't so much of an issue for the camera. While these are _comparatively_ small, they are still fairly big lenses in absolute terms.

And it won't be tack sharp on a 24mp APS-C sensor. It may be reasonably sharp.

Also - just to put this into the grand scheme of things - the Olympus 300mm f/4 - while more expensive - will eat those two RF lenses for lunch.
The effective output quality will be higher - and unlike these RF lenses, the Olympus can take full advantage of Dual-IS - while also being an equivalent stop faster.

Of course, the argument won't matter if you own an R camera. However, in this specific cases, the equivalence game fights back.

Out of curiosity, I looked up the price of the Olympus lens you mention. $2790. That is more than both the 600mm and 800mm lenses. Of course, the Olympus  needs an 1.4x TC to compare to the 800mm lens. That adds $349, coming to $3129. So that now 840mm f11 lens will eat that Canon 800mm f11 lens for lunch? For a mere $2229 more? And of course it is not a stop faster than the Canon 800mm f11...

actually, f4 plus the 1.4x TC should be f5.6., or not? Or I guess you included the equivalence? (f4 => f8, plus 1 stop for TC).  And anyway, if the 800mm was available on an APS-C camera, the factor to MFT is only 1.25 (not 2), so the 300 is a 375mm, you'd have to add the 2x teleconverter to get close. I guess Klaus has to do a comparison ;-) I hope he has a 50 meter room, or whatever is needed ;-) 

In an utopian world, one has a FF sensor with high pixel density (diffraction limited). You put any lens in front of it, even MFT, or EF-S, and the pixel density is high enough to capture the best picture. However, the FF sensors will remain more expensive than smaller sensors for a while, as far as I can see.
I had once the crazy idea, to have a body with two sensors. A normal, say 20Mp FF sensor, but then you can flip in a mirror that redirects the central portion of the image onto another smaller sensor (say 1 inch) with higher pixel density for the long reach images. While weird, I guess it would have been less weird than the Ricoh GXR...