Using 2133 ram on mid macbook 2012
Realistically the speed difference between 1866 and 2133 isn't going to make a significant impact on your overall system speed. It's a 266MHz difference but this does not affect overall performance in the same way a 266MHz CPU speed bump would.

You say you are running virtualised environments, so upgrading to 16GB - whatever the RAM speed - will have more of an impact than the RAM speed itself.

If you're looking for the maximum performance from your setup you do really need to be running an SSD in addition to the 16GB RAM, if you're not already.