Make sure you're using Legacy mode boot plist. No I haven't tried Sierra yet on that system.Have you tried this on your original build? I cannot seem to get the install to work. I have exactly the same components as yours.
PB3 still has no full OOB support, like earlier betas. Only framebuffer is OOB, acceleration needs Hacks to work & might still be buggy.Sweeeeet
than I will Order my 480...
maybe even two of them to use as an external GPU for my editing system (need to do some research on how to make it work)
thanks a lot for communicating your experience.