trident z rgb 16gb (2x8) 4000mhz

I am planning on upgrading a few components on my PC, The CPU of choice is Intel i7-8700k *which says it supports DDR4-2666, the motherboard I'm intending on using Asus ROG Strix Z390-E *which supports DDR4 memory at speeds of: 4266(O.C.)/ 4133(O.C.)/ 4000(O.C.)/ 3866(O.C.)/ 3733(O.C.)/...