Both M.2 and 2.5" SSD models push the same 560/530MB/s regardless of the connection (R/W on a WD Blue 1TB 3D NAND for example), pretty much all of the SATAIII M.2 cards max out at 560/530MB (as do the SATAIII drives).
ARC cache is in memory and can be extended to L2ARC cache on a SSD as a read cache, very curious to see how this performs on the Helios64 and if the 4GB of RAM is suitable. I pre-ordered BTW, so I guess I'm going to find out!
Maybe a little ambitious, but my goal is an Armbian\OMV\ZFS\Docker system.