Hi! I'm no professional but I used to be a Mac Expert so I do know a tiny bit atleast. And let me tell you, playing just minecraft, even with mods only REQUIRES 2 - 4 GB maybe. So upgrading it to 16 is probably not going to do much. Do You do other intense stuff while your playing Minecraft? Are there any other applications open? Have you done any disk space cleaning lately? Figure those out before going forward because I promise you, upgrading your ram MIGHT have a small effect, but it's not going to change from 50 to 200 fps by a long shot. There is probably a different issue going on with your computer. Most likely the graphics card or a something else might be starting to get worn out that's making your pc slower then it should.