Check the timing tables in CPU-Z. There should be a couple of JDEC tables listed in the memory section as well as XMP settings. Make sure that the RAM is reporting that XMP is indeed 1600MHz opperation (800MHz listed).
If the XMP table lists 800MHz and you have it enabled then you'll need to update your BIOS which is available on MSI's website under your MB's page. If that still doesn't work, or you've confirmed you have the latest version start a support ticket with MSI let them know what's up. They can send you a beta BIOS too if needed.
I upgraded the BIOS (after a fair amount of reading...) from 1.12 to 1.17.
Strange thing. Before the BIOS upgrade, I couldn't actual edit the XMP to enabled in the BIOS settings. Depending on the boot, it would be set on its own to Disabled or Enabled. Didn't make a difference though, as the memory was always showing 1333. It was a read-only setting.
After upgrade, I was able to freely edit it. Did so, and the memory is now 1600 in CPU-Z!