MacBook – run 2 16 Gig ram in the MacBook Pro (Retina, 15-inch, Mid 2015)

1067 Mhz DDR3 is what you care about. You can do it on your own. Apple provides instructions on their website and you can find it on many places too.

For installing the RAM you just need a screwdriver small enough for the screws on the bottom of the computer. After that ground your self or use plastic gloves and take out the old memory put in the new and you are done.

Manufacturers don't matter as long as the memory is 1067 Mhz DDR3 for laptop. Its better to go for well known brands I guess such as kingston,crucial corsair etc.

I bought a Crucial Memory Kit 8GB from amazon for about £35 same type of memory with the one you need and it worked perfect.

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.