FPS boost? Not normally. Just in the cases of when the thing you want to render doesn't fit in to 8gb of ram, but is pre-loaded in 16gb. Then the judder of reading in new assts from the HDD is reduced. Ram doesn't render anything, it's a storage space.
You will load games quicker, and the desktop will feel smoother when you come out of the game as a bunch of crap isn't being read back in from swap. Games that you regularly play will start to load a whole lot quicker as Windows learns what you like. 16gb gives you a better moment to moment experience, not higher frame rates.