If I bought ram, I'd buy 2133 MHz 9-11-10-27 @ 1.5V. Even that can be hard to achieve with some CPUs.
That said, the high-bin memory offers the greatest flexibility when it comes time to OC...you don't need to run the rated timings...
Every 2666 MHz set I have owned ran 2133 MHz 9-11-10-27 @ 1.5V, so that's a safe bet, and it has overhead for benching when that's needed.
I have also found that the 1866 divider is slower for memory write performance than 1600 MHz, so it's either 1600, 2133, or 2666 MHz for me, and what I end up using for 24/7 is all about a CPUs capability, and which provides the best results with the least amount of heat and voltage.