Canon EF 100-400mm f/4.5-5.6 USM L IS II - Full Format Review / Test Reportwww.opticallimits.com
It is not weak per se, but there are very few zoom lenses that are strongest at the long end.
And of course the teleconverter comes on top of that.
The EF 100-400 II @ 400mm on the 5DsR was so sharp at infinity it made my eyes bleed, even handheld. If the 100-500 IQ @ 500 matches or exceeds the 100-400 II IQ at 400mm, then it's going to rock. Given the performance thus far of the RF L lenses, I see no reason why this would not be the case.