This morning, the local store got in another two copies, and I was hoping to buy one after testing. On the 5DSR and using a tripod, one copy was so so and the other was superb optically, from centre to edges at 400mm. But even at very high shutter speeds of about 1/4000s, the image quality deteriorated a bit hand held. At speeds of 1/160s and 1/250s, there was significant camera shake in the standard OS mode, which improved somewhat on reprogramming with the Sigma dock to dynamic OS (which I find best for my 150-600mm C). It was still noticeable and unsatisfactory. Cameralabs had noted in its review: "Its biggest let-down is the missing tripod collar and most importantly the disappointing OS performance: it is not very effective around 1/100 sec and 1/50 sec which is a critical range of shutter speeds for such a lens."
I thought maybe it was because they had used a Nikon 810, which has problems with some Nikkor lenses. My 100-400mm II has no IS problems at those shutter speeds. So, disappointingly, I had to return both copies and will be travelling with the heavier Canon 100-400mm II tomorrow.