Using memory with a higher MHz rating?
The processor is hardwired to talk to the memory so it will throttle memory down to the speed it can talk at.

Installing 1600MHz RAM will just run at 1333MHz in that model MacBook Pro. There is no impact on battery life, performance, reliability etc.

I know there was an issue on some models where it would not boot with 2 modules of higher speed but I believe it is fine with the new models. There are utilities you can get to change the memory speed (on some modules) to get around this issue. I am not sure if this is the case with newer models.

The last time, I looked the price difference between these speeds of RAM was so small, I would just get the 1333 models for safety.

(MHz = clock speed)