If you have an issue with the machine, apple will potentially point at the below spec memory as an issue - sometimes even if it's not. While it's considered user replaceable expectation is it's the right spec.
While it should run, it'll hold the bus clock down. Peak transfer rate on 1333 is 10667mb and on 1600 it's 12800mb. And you effectively lose 267 transfers per second in the bus.
From there it's a matter if the latency and CAS timings are the same. If it were me, I'd go with the correct spec, but what you have may well work.