Cool thread - I'm also looking at upgrading my mid 2014 MBP's 250 SSD ... Has anyone tried the longer Sintech adapter (the shorter one seems to have some issues)? If so, is it available in South Africa somewhere?
If this works, then I'll opt for a 1TB drive - currently I can only find some for around $400 that fit my laptop - online here: https://www.ifixit.com/Store/Mac/OWC-Aura-Pro-X-SSD/IF123-123?o=3 (no adapter needed).
I bought Intel 660P 2TB m.2 + adapter from amazon and installed it successfully on my late 2013 MBP. This m.2 SSD is lower than average performers but it works for my usage. $200 for 2TB m.2 SSD, $30 for tools and adaptor(long one). You need to add shipping. It was simple to do the upgrade. Check out the youtube below to see how it is done. The author of the video also put the links for the parts used in the description.
This is after I have installed Intel 660P 2TB m.2 on my late 2013 MBP. It is pity that RAMs are soldered. So it is not possible to upgrade RAMs. :-(
I have installed my new SSD with the longer adaptor and it does not give me any issue. However after upgrading my SSD from 256GB (Sandisk MBP original) to Intel 660P 2.0TB. The boot takes longer. It can be my economy grade Intel SSD.
I could not find them locally, so I got them from amazon.com. It took them 1 week to prepare all my package and another 1 week to deliver them to my door via DHL courier. I took regular courier option. However as they took too long to ship them out of US, they used the faster option at no extra charge. Very happy with their service. If you can't source your items locally, amazon.com can be your option.