A pity that there are no 100% crops in the Chinese link. The measured resolutions are interesting. At 600mm and f/6.3 it's very soft. But, at f/8 it is pretty good and f/11 excellent.
There are 100% crops of swans and the model eye.
Anyway, I recall that the initial promotional images were shot at f/9.0 or so. Probably the sweet spot for the 600mm end is around there.
Sorry, I should have said informative 100% crops. There is precious little detail on the swan's face or neck that one can see to judge whether it resolves it. We need to be shown a bird with real plumage, not a blurred whitish.
5DS R, 7D II, EOS-M, Powershot G3 X, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M 18-55, f/2 22, 11-22