The time came to upgrade my trusty MacPro3,1 to macOS Sierra. As many know, El Cap is the latest build of OS X macOS that supports this platform. How to a MacPro3,1 on Sierra is well-documented, nevertheless this blog is half just a diary that I use to re-trace my steps later.
After making a backup image of my system, I used the guide here to create a modified Sierra installer. I then simply ran an in-place installation and sure-enough everything seems to be working. During the install, I swapped back in the original (coal-burning) video card. Afterwards I swapped back in my current GPUs and all was well. So far Sierra seems to run as good as El Cap, maybe.. Just maybe even better.