I think the 800mm f/5.6L has always been touted to have stellar sharpness and overall IQ.

The downside is it's size and weight.

The 600mm is not too easy to deal with but the 800 is not hand-holdable for 99% of people and an even bigger challenge to travel with.

I think the 800mm sales really dropped off when the new versions of the 600mm and the teleconverters hit the market.

Most all the reviewers noted the new 600mm plus the 1.4 TC version II or III produced images of equal sharpness to the 800mm. So you get equal performance with less bulk and increased versatility .

I am hoping they bring out an RF 600mm f/4 DO lens!!!