Using my Mac Pro 2,1, I ended up installing a fresh version of Windows 8.1-64 using BootCamp mode (GUID with MBR emulation) on a left over SSD I had. I ran into a few issues (the partition was not set to active plus I needed to run other start up repair), which I resolved using iPartition and booting into VMware. I installed the latest version of BootCamp drivers by running the install from an administrative command line using the buried setup.msi command. I uninstalled the Apple ATI driver and let the OS pick its own version.
I then upgraded to Windows 10 actually running it under VMware. A lot churning later, I was able to do both VMware BootCamp and native boot. It is using the latest ATI drivers provided by the OS. I did have some .net issues from Catalyst remnants, which I resolved using first the catalyst uninstall and clean tool, then an online tool to clean up outdated framework.
Everything is now functional on this system in Windows 10-64.