This message was posted on behalf of Intel Corporation
Hi Freesk8r, Thank you very much for joining the Intel® processors communities.
The memory controller is located on the processor itself, so the processor is the one that indicates the max memory speed to use for the PC to work properly. As you mentioned previously, the Intel® i7-6700K supports DDR4-1866/2133, DDR3L-1333/1600 @ 1.35V, so, even if the motherboard supports higher memory speeds, it is not recommended to force the processor to run at speeds it does not fully support, because the PC might work fine for a while even for months, but sooner or later there might be performance issues and in some cases the PC will not work at all. Some customers purchased the memory you mentioned above, so what we recommend in those cases is to adjust the memory speed manually in the BIOS to run in this case at 2133MHz as max. Some motherboards adjust the memory speed automatically to that value. So, for us there is no reason to buy DDR4 2400/2666 since we recommend to set the memory values based on the processor specifications.
Any further questions, please let me know.