Would Upgrading to 16GB RAM be Beneficial?
Looking at your memory usage you don't need more RAM.

To answer your questions:


Apps may open faster if you are running lots of heavy duty apps, like Photoshop, Xcode, etc…
I doubt you'd see much of an increase in the FPS in games. That depends on the video card and how well the game is written.
Overall performance increase would probably be minimal. The biggest bottleneck is if you are running off a spinning disk, upgrading to an SSD would probably be the biggest performance gain, depending on what you use your computer for.