Well, success! The adapter I mentioned in my last post fits the Samsung 951 and an Early 2015 13-inch MacBook Pro like a glove. The below results are for the Early 2015, older models may be slower.
The 951's screw notch even lines up perfectly with the screw hole for the original SSD.
Sierra installs noticeably quicker; AJA System Test averages 1320-1356 MB/sec read, 1235-1282 MB/sec write!
Now testing FileVault2 compatibility.. looks good so far. Wow, only about 20 minutes to encrypt a fresh Sierra install..
Under FileVault2, AJA System Test results drop a bit.. 1251-1269 MB/sec read, 955-1064 MB/sec write.
After all that, SSD doesn't feel burning to the touch, it's a little warm but nowhere near uncomfortable, will update when I get a working infrared thermometer.
So yeah, now 2013-2015 rMBP's have another possible replacement for the SSD.. but unfortunately pretty much for 256 GB as the 512 GBs are hard to find/pricey.
Note: original Apple OEM 256 module only gets 750 MB/sec read and 635 MB/sec write on same machine under FV2, so it's a win win even if you go for added security.