RAM Type: PC3-10600 DDR3 Min. RAM Speed: 1333 MHz
Details: Supports 1333 MHz PC3-10600 DDR3 SDRAM.
Standard RAM: 4 GB Maximum RAM: 16 GB*
Details: 4 GB of RAM is installed as two 2 GB modules, no slots free.
*Apple officially supports a maximum of 8 GB of RAM, but as first confirmed by site sponsor OWC, this model actually is capable of using up to 16 GB of RAM with two 8 GB memory modules.
although Apple sold and marketed the 2011 MacBook Pros as using 1333MHz memory, they are, in fact, capable of utilizing 1600MHz memory, just like the current crop of MacBook Pros.
Lloyd Chambers of Mac Performance Guide did some in-depth testing and found that a 2011 MacBook Pro with 1600MHz memory saw a 2% average performance boost over the same configuration with 1333MHz memory.