> As far as I know, a M.2 SSD and an SATA SSD use the same memory chips.
> Why would a M.2 SSD connected to a SFF-8643 port be faster than an SATA SSD connected
> to a SFF-8643 port?
Because SATA itself is increasingly the bottleneck for I/O as flash gets faster and things like command queueing come into play even on desktops. M.2 gives you 4 lanes of PCI Express, which is quite a bit more bandwidth.
That said, connecting one to an SFF-8643 SAS port probably negates a lot of that advantage.