Did you ever find a solution? I also do not have the 8700k, so im entirely unsure if the majority of the RAM questions and answers are even valid for me without.
I haven't upgraded yet because this motherboard is a **bleep** headache.
I upgraded to 64GB (2 x 32GB sticks) from Corsair Vengence LPX and it booted at 2133. I went to performance settings in the BIOS, XMP1 was disabled. I hit the optimize button on the right side and it refreshed with XMP1 selected. I saved, rebooted and I'm at 2666. In Ubuntu, ram speed confirmed at 2666 as well, seems stable, fast too - as you'd hope.