Will my 2666 RAM run at its rated speed?

No. Your 2666 RAM will run at 2400 MHz because the CPU’s memory controller maxes out at 2400. You’ll need to enable XMP in BIOS to reach 2400 instead of the default 2133. The extra 266 MHz won’t benefit your i7-7700K, and you’re not losing performance—just make sure XMP is on.