Was the same to me but I have successfully "overclocked" them in the BIOS. I only have changed the clock speed to 1333MHz and left everything else on auto. Now my Proxmox test installation shows 2666MT/s (via dmidecode on command line).My memory modules are 2666, too and they are running on 2400.
I'm already using new 1.20 BIOS.
I've installed 2x Crucial CT16G4RFD4266 (which are not on Asrock Memory QVL but the bigger 32GB version).