Thanks for your sharing.
It's interesting to know that Windows can boot faster on a PCIe SSD. Because only very few people actually report here about successfully boot Windows from the PCIe SSD. So, my mindset still stuck at "how to boot Windows from PCIe SSD", but not "will it boot faster?"
And TBH, I don't know the technical stuff behind a VM. But it seems the VM itself is actually a single large file. If that's correct, then I can see it can benefit a lot from the PCIe SSD (AFAIK, handling a single large file is the strongest area of the PCIe SSD).
I think use the NVMe in Windows as storage / scratch disk only (not the boot drive) should be OK. It's a bit like we can't boot via a USB 3.0 port but can use it without issue once boot to desktop.
It certainly does work, but it wasn't easy to work out how to get it there. Some people seem to be able to install to it when booted into the Windows EFI installer. However, I can't. It demands a driver, despite it being driven by a generic Windows driver when running. If the SM951 is formatted, it will even let me browse the drive to look for a driver.. to get it to work?! Nuts.
I had to install Windows on it initially on a spare desktop I have. Ironically, though my spare machine can see it under the Windows installer, the BIOS doesn't see the SM951 as a boot device, so the setup never completed past the initial install. Once I realised this, I moved the SM951 and the install DVD back to my Mac, and astonishingly the installation carried on!
You're absolutely correct, the VM is basically one giant file. It's obviously a little more involved than that, but it's very impressive seeing this tiny stick write that fast. Especially in a machine from 2010
I know that there was that generic NVMe driver someone wrote for OS X, but it stopped working. But since Windows doesn't have that limitation, I wonder what would prevent Windows using the Mac Pro from using it. May well be hampered by firmware on the logic board not seeing it at POST I guess.