OK; it's officially not a bug. SM support have explained that SreeD is for those who can't use OpenGL and that OpenGL is better. SreeD is also not designed for use with Retina. So, all in all, you're ok on Sierra with full resolution and using OpenGL. If, like me, you don't like the heavy shadows in OpenGL Preview then you can always switch full res off and use SreeD. I had another run with Sierra earlier today and everything seems ok, though note that I don't yet use, and haven't checked, Face, Hair, Cloth and Set-Up rooms. If you need them, then I suggest you get yourself an external drive, stick Sierra on it, and then check them and report back. The SM support fellow said they'd found everything running smoothly.