I do know that if it's not supported by the underlying filesystem if falls back to the default. What I don't know is if it's faster to copy than a regular copy

I do know that if it's not supported by the underlying filesystem if falls back to the default. What I don't know is if it's faster to copy than a regular copy on a ram tmpfs - since it may be essentially be doing "nothing" - updating some pointers instead of copying stuff in ram. So if your workload is limited by copy not execution speed it might be worth testing. However at that point you're running on a regular FS so if you're trying to benefit test speed by having all your regular postgres operations on a ramdisk, that part will end up being slower.