If you go 4X mode you get all those lanes bonded, similar to turbo mode on the 6700K in single core performance, the immediate trade off seems to be loosing SATA ports 5 and 6, which I don't use anyway. What worries me is enabling M.2 support in the next field drops SATA ports 1-4, which calls to question what impact that has on the 4X lanes in the former change if I leave the latter set to SATA Express. If it sticks the m.2 drive at SATA 6GB speeds, then why bother.
I'm still researching.
My 250GB EVO's 850's are plenty fast, but I'm looking to re-purpose those for content over the current spinners. It may all be overkill, but I'm in the process of upgrading hardware between machines, so now's the time.