So far 4.28.0 is working nicely on my system, although after switching external monitors a couple of times, I did notice a massive slowdown in the framerate obtained using optirun glxspheres (ie using the nvidia card via VirtualGL). It slowed down from 150fps after booting to only 60-70fps. (Note that just using glxspheres by itself gave ~59.1 fps instead of ~59.6 fps, so there wasn't a big difference there).
Restarting unity didn't help, but a reboot did. Might this have been an issue in compiz rather than unity?