You might want to try the following and see if you can boot at 2666 MHZ (maybe aim for the Moon first instead of the Sun).
1) Make sure memory chips are seated all the way and plugged into the red memory slots on the MOBO
2) Make sure you are running F6G bios
3) Set XMP to OFF
4) Set Memory Multiplier to 26
5) Set Channel-A memory timings to Advanced Manual - with the following timings (16-18-18-18-36)
6) Set the MEMORY voltage to 1.35 (manually - instead of auto)
7) Save the settings and reboot
You'll know when the MOBO takes the ram settings as it'll reboot on the first try (afer saving, and rebooting) instead of doing the 3X bootloop (with the beeps), then falling back to 2133 MHZ as a failsafe on the 4th boot.
On my motherboard, if I don't set the memory voltage to 1.35 - I get the dreaded 3x bootloop (with beeps), then falls back to 2133. My RAM is rated at 1.20 volts @ 2666 MHZ per SPD.