When I updated to Windows 1803, task manager reported my ram speed as 931. Which Im assuming is what you mentioned above and was really running at 1866. I updated my BIOS and the task manager was reading correct at 1833. But I also thought this was low, and when i tried enabling XMP to 2666 which crashed my system because that speed isn't supported with 4 DIMMS on my MB.
I just recently changed it to XMP 2133 and have had no issues so far and ran a few MemTests and all seemed well. My MB says I can do up to 2400 so I might try that next. However what would be the performance jump from 4x4 16GB @ 2400 vs 2x8 16GB @ 3200 I guess is the real question now.
EDIT: Added CPU-Z shot of Memory, I'm seeing max bandwidth at 2132, so I'm assuming 2133 is the max I can go atm?