Thanks for the answer, Kevin.
Regarding the 1st point: well, neither drive is SATA. As far as I understand one is PCIe AHCI and the other one is PCIe NVMe.
Regarding the capacity vs. performance: Samsung does quote a difference in sequential write performance, taken straight from your review:
512GB - Up to 1,550MB/s
256GB - Up to 1,260MB/s
and for the AHCI version, also taken straight from the SR review:
512GB - Up to 1,500MB/s
256GB - Up to 1,200MB/s
This makes it even more astonishing that the NVMe version can often keep up with or beat its AHCI cousin. I know it's not easy to comment on drive sizes you don't have in-house, but this doesn't mean you should ignore known differences either.