I had big issues with my flashed 980 after the sierra upgrade. After the upgrade my mp5.1 was booting up with the apple logo and then all screens went black and the fans run very loud...Only a cold shutdown was possible. I tried everything... Clean Install on a external hd, but the same behavior happened. Under Bootcamp everything was ok.
I was confused, cause with elcap everything worked normal. So i started my oddysee in the web to find out, what the problem was... After a very long search in a lot of forums, i found a little post in a hackintosh forum about black screen with dual displays with the advice to use only one display. I thought, ********, 2 displays worked in 10.11, so why not in sierra? But at last I tried it and bam sierra booted up... I was so glad, that my mp wasn´t broken.
But how can i use 2 displays again... Another little post gave me the tip to use displayport instead of dvi. I bought a dp to dvi cable for one Display and hdmi for the other one. And Bam it worked... Sierra with 2 Displays and till now everything works fine...great...