[davidmanze]

"Luckily, one can do other things with lenses than make mostly boring BIF images..."

I agree!  That's why I am a fan of Dave's bird in flight photos.  They are pure art!

But I guess what you meant was that even if the RF 600/11 and RF 800/11 art not state of the art on an EOS R5, they could well be excellent on the EOS R6 since it has a DLA of f/10.6 which suggests that f/11 can be expected not to have any real world DLA issues.  But...how the EVF may perform could be an issue in either for either camera.  It's the sort of thing where I am not sure anything less than trying one's self will clarify it.

For me I would be more interested in the R6 because if one were "stuck" with performance resembling the EOS 1DX Mk. iii, for 30-40% of the price one would not be too disappointed.  Unfortunately, I guess there is a reason for the price difference.  

What I would like to try would be the R6 and the 800/11 + 1.4x tele.  The logic being, I can not come close to anything with this focal range that will work on a DSLR.  Would it be usable on BIF photos?  Well, maybe.  Migrating birds that fly very high are of great interest, because many species can only be seen when they are migrating.  But other than that there are lots of wildlife possibilities.  Super tele is fun.  Pushing the limits is fun!

The EVF, IS, and Focal length interaction issues have been stated pretty well by various factions.  The key, as was mentioned, is at this price range, you almost have to try it out if you can!  Only on the R6, I would think.  I'd want to!

 Thank you Arthur ! ........
  Pushing the limits can be fun, yes  ...... but it can also be very disappointing ...... doubly so if you have just spent a few grand finding out ...... 
  
  5,000 ISO is way too high for birding  ..... I don't like going above 400 ISO in general, 800 if I have to ....... 5,000 never! .... featureless skies reveal lots of noise and need plenty of NR ...... and there goes your birds detail ...... and birds look like they're made of cardboard

 So far the 800mm F11 images just look to be not sharp from what I've seen on the Canon RF forum ..... and noisy (mostly) ..... if you add into the mix a 1.4TC giving 1,120mm ..... you will get less sharpness and even more noise ...... which equals unusable images and no advantage of reach from the APSc sensor. 
   For reach you have to stick with APSc ...... I have the FF D750 and rarely use it, birds in the VF become very small and seeing what the bird is doing, difficult ...... then you go home and crop all that good stuff down to less than the APSc format anyway.

 Personally I think you are the right track with the Canon 90D, maybe a 7DII is better (if you want to stick to Canon)....... and get yourself a lens that goes to 600mm and a tele-converter to go with it .....

   Or, the D500 plus the Nikkor AF-S 500mm F5.6 VR FL lens plus the 1.4TC III converter ..... that lens is super sharp ...... super compact and light and works well with the TC 1.4III ...... that will give you 1,050mm FOV and pro quality images ....... that lens is a miracle!

See here:
https://imaging.nikon.com/lineup/lens/f-...r/spec.htm

  Last word ....... the laws of physics have not changed .......
..... these two new Canon lightweight tele primes are testimony to that, they are there for occasional use for those who are not serious 
about wildlife photography.

...... and maybe Klaus could confirm it's sharpness from the links MTF charts?