I changed my ram to be in slots: DIMMA1, DIMMB1, DIMMC1, DIMMD1.
No change to the benchmark speeds as reported by Memtest86 and Memtest86+. However, I did temporarily install Windows 7 Pro and ran PCMark 8. My memory scores (gross scores, bandwidth, latency, etc.) met or actually exceeded similar single socket 612/X99 chipset/DDR4 boards, so I think everything is performing where it should be.
Based upon the ~50Gb/s bandwidth seen in PCMark, I'm guessing that Memtest86 is just reporting 1 channel of DDR4 ram, which would make sense as it showed 12.5Gb/s. Multiply that by x4 channels = approx. 50Gb/s.