I have a MacPro 2009 reflashed as a 2010. I am considering to install a Tempo SSD PCIe card and then put High Sierra on it. This card is advertised to allow booting on it: would anyone have first hand experience with that? The background story is that my boot drive is a RAID 5 Softraid with 4 disks (the four internal slots of the machine) and I have just discovered that High Sierra can't be installed on a RAID. So if I could have put one SSD on that PCIe card and boot on it, the upgrade path would be very simple.
I successfully installed a bootable High Sierra system on a Samsung EVO 860 (1 TB) mounted on that Tempo SSD PCIe card I asked about. I kept the RAID 5 alongside for the big important data.