There's no such thing as a RAM maximum for performance. You can trade processing solutions with memory solutions often enough. eg. Calculate dynamic LODs using processing, or store 100 different precalculated LOD models and just fetch the required one. Calculate procedural materials in realtime, or calculate them once and store the result.
I didn't say it was "a RAM maximum for performance" I said "Maybe it's just that 5GB is all that's needed for games on a console with less than 2TFLOPS"
Meaning that they figured that 5GB was enough for what they would be doing with the PS4 games & thought that it would be better to reserve the other memory for the new models in cause they want to do other stuff with the OS.