The memory QVL is at https://dlcdnets.asus.com/pub/ASUS/mb/LGA2011/Rampage_V_Extreme/R5E_QVL.pdf?model=RAMPAGE%20V%20EXTREME
My understanding is that Haswell-E/X99 gets pickier after 2666MHz, so I put a Corsair 16GBx4 CMK64GXM4A2666C16 kit in my GA-X99-UD4.
It seems like your board could a 8GBx8 kit of CMK64GX4M8A2800C16 for 64GB at 2800MHz.