Yep, that’s the NVMe version of the SM951, which Macs won’t support. You need to get an SSD that’s AHCI compatible. Maybe where you bought it from will let you swap?
Bugger all really. Not worth the cost vs. a traditional SATA SSD I reckon - unless you’re after insane IOPS for some reason.
Not really. I also like that the standard form factor will be re-usable in an external case easily in future or in a standard PC etc. Don’t like investing in device specific hardware. Was just curious to know how much faster. 
For what it’s worth - the M.2 standard is pretty popular. Heaps of laptops have M.2 slots and many PC desktop motherboards have an M.2 slot as well
Nothing wrong with the 2.5" drive though - it’ll be useful for a long time still.
Good to know. Could be handy if I end up back in the Hackintosh world once this Mac Pro 3,1 is no longer able to run latest OS. Really hoping Sierra runs with no real hacking needed (i.e. a basic target disc install vs new kernel). Failing that, I can dream about Apple releasing a real Pro desktop right? 
We’ve been dreaming for years, so, yeah.