Officially, the memory speed of a early 2011 MBP is 1333MHz (cas latency 9) because the that’s the CPU:s highest supported memory speed. Memory voltage is 1.5V.
And, officially, the max memory is 8GB (2x4GB).
So, if you would follow that, the best you could do would be 2x4GB 1333MHz.
It will however work perfectly fine with 16GB (2x8GB).
It is also possible to run 1600MHz (a.k.a. PC3-12800) memory, even though that’s not officially supported.
The thing is, most 1600MHz modules run at CL11, which will mostly negate any gains from the higher frequency. So, what do you do?
You go for 2x8GB 1600MHz CL9. Nowadays, these can be found, even the DDR3L type, at decent prices.
Suggestions? Kingston HX316LS9IBK2/16 or G.Skill F3-1600C9D-16GRSL, which is just that: 2x8GB 1600MHz CL9 DDR3L. Works perfect in a early 2011 MBP.
This will have a performance gain: My benchmarks showed a +25% increase in memory bandwidth with 2x8GB 1600MHz CL9 over the stock 2x2GB 1333MHz CL9. Kinda cool :)