M2_1 only has the ability to select Auto/PCIe/SATA
M2_2 is the one that you can select X2 or X4 because it shares bandwidth with SATA ports.
M2_1 will disable SATA port 1 if SATA is enabled.
I set mine to PCIe as thats what Im using and what I will always be using.
As for raid I've tried it and you still get bottle necked by the DMI link. You get a nice bump in write speeds because they are still well under the bandwidth limit but the gain in sequential read speeds inst very promising.
As for Optane, this is more targeted at systems with spinners as cache and doubling the drive access time of a mechanical hard drive which still falls well short of a 960 pro. Other than that it shows the same performance gains across the board as using ram drive with the gain that its non volatile. It also has the same restrictions that those gains are only for shallow que depths.
Im going to wait until later in the year to see where the M2 market takes us along with the X299 chipset and skylake X. As of now the drives are about as fast as they can be without bottle necking downstream of the PCH controller. Intel is to release an optane drive later in the year but I suspect as before by the time they get it to market the competition will one up them yet again.
As for drivers, the windows drivers 8.1 and later work just fine. No gain to be found with Samsung's drivers.

“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein