I think they do not go over 1333 mhz ( or else apple would sell an expensiver model with 1600 memory too), the real-world performance difference between 1333 and 1600 is subtle, tighter latency is what you should look for if you want more noticeable improvements i.e a 1333 CAS 6 or 7 kit.
You could also get 1600+ kit and try it, worst case scenario is that it won't boot until you set it back to 1333 or lower.