should be, you will just need to set the speed to 1866 and possibly faff around with the timings.
Yeah but it would only run at 1866 unless you change the setting onboard and overlock the ram to 2133. As long as the board allows for you to overlock your ram. You will not see a difference between 1866 or 2133. Honestly you could go down to 1600 and not see a difference. Only time you see a difference is with AMD APUs. I would find a 1600 with the lowest Latency you can find. 1600 with a timing of 8 would be fantastic or a 1866 at 9. G.Skill makes sniper gaming 1866 with a case of 9. Real world gaming might see 1 or 2 frames better.
Actually Yes it is. It's not listed on NewEgg nor the Website but it is listed in the user manual under 2133 MHz compatibility
FYI the MOBO officially lists it's support up to a few select 3200 hmz kits.