I've only ever used 5GB RAM tops. You should never have that many applications open to need more than 8GB because there is no way you can use that many applications at the same time.
I use up all my 6GB of RAM on any given day. I usually game, video edit, and currently learning other programming languages. The max you can get is 16GB of RAM for the m14x r2, so I'm planning to get that much. No matter what people say, "that you will hardly use that much," it will still speed up your PC dramatically.
Ok, if yo actually need to use that much RAM then by all means, get it. It isn't going to speed up your system, just allow you to do more at any given moment.