What an awesome thread. Unravelled a lot of confusion for me, and led to the discovery of DriveDx, which is pretty awesome.
My story: Bought a 2nd hand 2013 Mac Pro a few months ago - the previous owner had put a 500GB Samsung 960 EVO in it, although I didn't pay any attention to what SSD it was until today when the Mac Pro started repeatedly getting kernel panics either shortly after startup, or during boot up. Solved by re-seating RAM and SSD and doing an SMC reset (stupidly all at once, so I don't know exactly which action solved the issue).
Until I looked at the SSD I had always assumed it was some Apple-specific pinout SSD like my Late 2013 rMBP has. 500GB was never enough for me but I'd assumed the only option for these 2013 Mac Pros was the OWC AuraX range (which are $$$$ for a 1TB), so I bought a 1TB Barracuda and whacked it into a USB enclosure, and use that to store some of my less performance-intensive audio samples, iTunes library, etc. I thought I'd get 500-600MB/sec out of it and figured that'd be enough - but it turns out you only get 370MB/sec performance via USB enclosures.
So thanks to this awesome thread I can see now that if I really want 1TB of storage at 1300+MB/sec I could upgrade the internal one to a 1TB M.2 NVMe SSD rather than this out-rigger approach I've used (and just knowing that I could be running it all faster, will probably drive me to upgrade it eventually anyway :-p ) ....
BTW, I'm also seeing DriveDx is showing the EVO 960 getting a bit hot when I run BlackMagic disk speed test - that's good to know, I'd better order me a heatsink.
Thanks to everyone for posting such a great amount of information, and thanks to the folks who were happy to be guinea-pigs. I also appreciate the focus on empirical data & answers.