I'm experiencing that the 4gb 1600mhz ram in my macbook pro 13" mid 2012 is way to little for the programs i'm running such as photoshop. I got 8gb of 1333mhz ram in my older macbook pro 13" 2011 and thinking about swaping the ram, so im wondering how much performance difference would it be between using the 8gb 1333mhz ram or buying a new 8gb 1600mhz kit?
Mixing two different speeds isn't a good idea. When you install a higher speed memory (like 1600MHz) with lower speed memory (1333MHz) then the higher speed will default to the lowest speed memory that is installed so getting a new 1600MHz memory is a viable option.
If you have single 4GB module installed then you can add another 4GB to the existing memory. Else you can go with two 4GB modules.
Click here --> for compatible 4GB module.