I got my 970 EVO 2TB and it works great with bootrom 140.0.0.0.0 and High Sierra. It just takes a little longer to show the Apple logo and actually start booting than with the SM951.
To anyone wondering how to clone/upgrade, here is what I did:
I previously had the SM951 with 10.11.6 El Cap on it and my home folder on an 850 EVO installed on a SATA III Controller. I then did a fresh High Sierra Install on an old 1TB HDD. From there I upgraded my bootrom to 140.0.0.0.0. I then put a second 1TB HDD in the Mac Pro and used Disk utility in the freshly installed High Sierra to clone the system volume from the SM951 to that second 1TB HDD. You could also use a bootable Stick for that but I needed full OS X because my home folder was on the other SSD. After 10.11.6 was restored to the HDD I copied my home folder over to that same HDD and booted it. After setting the new location of my home folder and a reboot, permissions were broken so I ran "diskutil resetUserPermissions / `id -u`" to fix permissions and afterwards everything worked again. I then upgraded then removed the old SSDs and upgraded to High Sierra on the HDD to have a backup if anything went wrong. Luckily the upgrade worked fine. Now I put in my 970 EVO 2TB and booted the clean High Sierra install again, which prompted me to format the new drive. I made a mistake here by formatting with APFS. After restoring my upgraded High Sierra Install to it (which took about 4 hours for ~600GB) it wasn't bootable. So I reformatted it with Mac OS extended (journaled) and restored again. Now everything work perfectly!
I already have an RX 580 here. As soon as the power cable arrives, I will upgrade to Mojave which might also change the filesystem to APFS.
To anyone wondering how to clone/upgrade, here is what I did:
I previously had the SM951 with 10.11.6 El Cap on it and my home folder on an 850 EVO installed on a SATA III Controller. I then did a fresh High Sierra Install on an old 1TB HDD. From there I upgraded my bootrom to 140.0.0.0.0. I then put a second 1TB HDD in the Mac Pro and used Disk utility in the freshly installed High Sierra to clone the system volume from the SM951 to that second 1TB HDD. You could also use a bootable Stick for that but I needed full OS X because my home folder was on the other SSD. After 10.11.6 was restored to the HDD I copied my home folder over to that same HDD and booted it. After setting the new location of my home folder and a reboot, permissions were broken so I ran "diskutil resetUserPermissions / `id -u`" to fix permissions and afterwards everything worked again. I then upgraded then removed the old SSDs and upgraded to High Sierra on the HDD to have a backup if anything went wrong. Luckily the upgrade worked fine. Now I put in my 970 EVO 2TB and booted the clean High Sierra install again, which prompted me to format the new drive. I made a mistake here by formatting with APFS. After restoring my upgraded High Sierra Install to it (which took about 4 hours for ~600GB) it wasn't bootable. So I reformatted it with Mac OS extended (journaled) and restored again. Now everything work perfectly!
I already have an RX 580 here. As soon as the power cable arrives, I will upgrade to Mojave which might also change the filesystem to APFS.