ISO 12233 Resolution Chart sample crops have been added to the Canon EF 500mm f/4.0 L IS II USM Lens Review
and the Canon EF 600mm f/4.0 L IS II USM Lens Review
Not surprising is that both lenses turn in very impressive results. Comparisons with the older versions of both lenses are available. The 600 L II + 1.4x III compared to the 800 L is interesting.