Using the higher speed RAM MAY allow you to achieve a higher OC on the CPU. Depends on the motherboard's memory divider settings. 1600MHz and 1866MHz RAM is OCed 1333HMz RAM (uses better memory chips (higher-binned) than regular 1333MHz RAM).
Or are you trying to OC the RAM? If that is the case, you may not have a lot of OC overhead on the faster RAM because Corsair has already used the higher binned (faster) memory chips in those sets of RAM.