I tried the RF800 for a couple of weeks. Decent IQ, but it was too dark. F/8 doesn't do it for me either. I don't like shooting slower than ISO400, but will go to ISO1600 if absolutely needed. Thus, I'd really like a compact 600mm f/5.6. The f/4 lenses are just too bulky, plus I can't rationalize spending $10k+ for a lens. Whoever makes that f/5.6 lens first--with excellent IQ--gets my money.
I'm with you all the way on that one - a 600mm F5.6 that weighed no more than the RF800mm would be very nice indeed.
I tried the RF 600mm F11 but for me the light weight was outweighed by the one stop advantage of my existing 100-400mm + 1.4x extender, especially as the zoom and extender, even when wide open is very sharp and not too heavy (and focuses much closer).
In the end I opted to supplement the zoom and extender by getting the 800mm. The extra reach is quite significant, and as I only intend to use it for distant subjects, the not-so-close minimum focus distance didn't really bother me.
I can live with the fact that the RF600 and RF800 aren't weather-sealed, as F11 restricts me to dry sunny days anyway. I can also live with the fact that Canon don't supply a hood for them, as I picked up a perfectly good third party hood for under $20. What I find much harder to live with is the non-rotating tripod foot.