The short answer is that it will depend on the model of your machine, and the RAM manufacturer you buy from — but I can't tell you any specifics on either.
Installing higher MHz RAM is a risk, and here's why:
My 1600 MHz memory didn't pass tests
I installed 1600 MHz RAM on my laptop and found that it failed the Apple Hardware Test, Memtest OS X, and the Tech Tool Pro memory tests.
These could have simply been faulty RAM modules, or just not 100% compatible with my machine.
So follow the advice below at your own risk.
"But it's compatible with my Intel processor"
Here's what I've found:
- Intel's spec for my i7 processor (2760QM on a Late 2011 MacBook Pro) says that 1600 MHz DDR3 RAM is compatible.
But does that translate for Macs? Yes according to OWC.
However, Macs are notoriously picky about RAM, and while 1600 MHz memory might be compatible with Intel, it doesn't meant it'll be compatible with the Apple logic board.
So find out if Intel supports it, but that is no guarantee.
Costs of 1600 vs. 1333 MHz
If the decision between two MHz-ratings of RAM comes down to price, take a look at the real-world performance increase and judge whether it's worth it.
It's likely that it may not be worth it to get the "faster" memory.
Also take into account the risks if it doesn't work: you'll have to return it (if even possible).
Is this a false positive?
Some people allege in various places that 1600 MHz will work because it shows up in "About this Mac" when installed:
And then "More Info" brings this up:
First, there is the question of whether it will pass memory tests (see the first section).
Also, I am unsure on how one can test if it is not being clocked down to 1333 MHz and the full 1600MHz are being used.
I am not sure whether showing up there means it works.