An analysis from NXGamer makes the hypothetical claim that with the SSD that Sony is using, they could move OS operations onto it and effectively give games like 15.5GB ram usage.
That is a pretty obvious idea, I'm sure Cerny has tested that idea with some games. They tested bc with the top 100 games and I'm pretty sure Cerny thought of adding that to the test suite to see how far they can go with stripping down the in-memory os.
Basically you keep the Kernel and the most used os functions in ram. At around 6G/s, loading a few megabytes of os code at highest priority setting (there must be a reason they have 6 priority levels in their custom controller) is almost unnoticable. My guess is 15 to 15.5GB usable ram is entirely achievable.