Seeing as I could not find any posts here or elsewhere that explained the poor performance of the Samsung 980 on my Macbook Air (mid2013), I decided to pull the trigger on a Kingston A2000. BlackMagic reports much more favourable write and read speeds:
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This is way more in line with what I would expect from a PCIe v2 x4.
As for the Samsung, well it is running the latest firmware so i'm not quite sure why it so was much slower than the Kingston. Guess i'll order an NVMe to 2.5" Sata adapter and stick in one of my other pc's.
I would be interested to know if anyone else has had the same poor performance with a Samsung 980 (mine was the 1TB unit) and the long Sintec adapter. At this point I would stay away from the 980 though..
Now to do a bootcamp install of Windows 10 and see if Windows crashes and I still have to do the SHIFT + F10 workaround.