[davidmanze]

"I think Arthur was looking at the reach advantage of APSc for those little birds and critters. "

Correct Dave!  I can remember back the the big splash the was made when the 70D came out with it's dual pixel focusing.  The thing that grabbed me was the claim that the camera could focus in very little light.  A reviewer claimed that in live view the 70D could focus in almost total darkness!   So I wanted to see if the 70D could "Focus in the dark".  I ended up buying one, and what I remember is that it really could focus in very low light.  The point being I was not really surprised that both of these new lenses can AF even with a 2x tele on them.

So what I was thinking was what kind of result you could get with an APS-C sensor.  I know you could adapt the FFD by using less extension.  At any rate I just bought the 90D recently.

...yes I know the bayonet mount part of the EF and EF-S are the same, and either one would work.  My idle question was that maybe the lens might not have to be majorly redesigned if used on, let's say a 90D in Live View Mode.  At least as far as image circle and enough room for the lens to fit on the camera.  So I am imagining 800mm x 1.4 tele x 1.6 crop  and thinking 1,792mm could be realized.  A birders dream!

The M Mount cameras are just too small...Which has been my complaint all along.  I want a form factor that fits large hands, and an APS-C sensor...And these new lenses!

I really can't afford an RF camera, so I this is just clouds in my coffee!

 Hi Arthur,
               Ah, dreams !!....
I see your logic and I think we have all dreamed about such combinations to get that frame filling shot .... 
...... I've tried the AF-S 500mm/600mm F4 + a 1.4X converter on both FF and the APSC sensors (1.5X crop)...... all readily available gear.
   I haven't the most recent converter being the older Nikon TC14E II, the MkIII version being said to be a little better ...... however, I found that sharpness tends to hold up only when you are close .... which is great for small birds . ..... 
 ....... as soon as you are any real distance things get soft again often with lower contrast/veiling and results tend to look a little like from a super-zoom bridge camera. .... if your used to sharp results you just won't be happy.
    ........ the magnification of air disturbance plus the tele-converter takes the hit ...... cropping is better and of course cheaper!

 Unfortunately, getting close enough is the bain of bird photography!