If the BIOS is set at 1600, 1866, or 2133, it will run at those speeds. The performance increase is small, but noticeable depending on what application is running.
When you first install the 1866 RAM, it will run at 1333. You just need to change the speed in the BIOS to XMP or the correct speed.
Right now, the price/performance sweet spot is 1600 RAM.