MacBook – What are the requisites for RAM in a MacBook

It could be the RAM is just bad. It happens sometimes.

But try to rule this out. Reset SMC and NVRAM(why not?) and boot into Apple Hardware Test to check the memory.

Also you may try to use new 8GB stick paired with old working stick. If it will be OK than don't try to buy same second 8GB stick it will not work. Though it does mean you can use this module with different fully compatible one it's better to send it back.

Anyway I would recommend you are buying DDR3L. It's a new and improved standard your model of MBP supports.

Getting a performance boost depends on what you actually do with the thing. Run Activity-Monitor (Applications>Utilities) to see how much of your RAM is used. Most Mac stuff runs well on 4GB, but if you run memory intense stuff (Parallels-Windows, Video Editing, editing large pictures with photoshop) you should see an improvement. But even if you don't need the RAM, you should not see a performance penalty (i.e. it may or may not run better, but won't run worse).

Unless your computer is already slowing down to a crawl with the current configuration (when doing memory intense work), adding 2x 2GB should be enough (giving you a total of 8GB).

So, given your configuration, I'd add 2x 2GB to the free slots and keep the memory that is already there in the other slot.