Running higher-rated RAM at stock speed?

Hi,

If I have a 2666MHz DDR4 kit, would it run at 2133MHz by default in an Asus Z170M-Plus motherboard, or would it overclock automatically by default?

I'm not sure I want to go down the overclocking route just yet but want the option later.

Normally it'd run @ 2133MHz till you applied XMP with the click of a button. It's officially termed "overclocking" but isn't really overclocking as in fine tuning of frequency, timings and voltage.

2666mhz will run fine on Z170, either via XMP or if it isn't a feature of the ram kit, setting the speed and timings manually.