UPDATE #2 (48 hours): Following on from
my post yesterday I've been running on completely stock Sierra install for 48 hours now and not seen a single glitch, nor have I been able to even provoke it into glitching like I could before. Login, logout all clean screens, running file migration wizard, quitting back to login screen and then logging in, all good. [NOTE: FiuleVault is turned off, that's the default on a fresh install and was off previously when I had glitches]
In case it was the discrete Radeon Pro 460 to blame I went into system preferences and disabled automatic graphics switching for most of today so I was running purely on the Radeon and no issues. I even confirmed I was running on the Radeon in system profiler which showed the LCD as connected to that GPU not the Intel 530.
I've run it all different resolutions for a few hours, I ran some 4K youtube playlists full screen for over an hour. I ran on battery and on mains. I ran some OpenGL benchmarks within safari. It's all silky smooth and no glitches.
So my next step - I'm going to install some (non-Adobe) apps via the apps store and test for another day and then I'll try with Adobe Lightroom and Adobe Photoshop CC installed and see if the glitches start to appear.
Long process but since I've still got my old retina MacBook Pro it's easy enough to continue with that for work and use new one for everything else.
Next update tomorrow.