I’m assuming this is a trash can mac pro? I haven’t tested that particular config, but on a macbook pro with either the 560 GPU or an external WX9100 I have good results in High Sierra or Mojave or Catalina.
For preview renders I would recommend using 1 GPU (not 2 + CPU). There is also an issue on macOS where the OpenGL interop between the device doing rendering and the display device. You might try going to the config.py in the addon directory and setting this line:
use_gl_interop = False
