As time goes by, MS
can optimize and reduce the resources used by their OS and apps. This is known.
They are saying that, in theory, they can give some back to developers. Of course, they could also use the same resources to improve Kinect or the OS instead.
Microsoft is claiming that they will go the first route, in which case we might get a 5% increase in gpu capability in a few years.This is negligible and can safely be ignored.
My theory is that this is some pretty effective marketing to push the story that X1 is closing the gap, which it can't, of course. If MS had a do over and knew GDDR5 would be available/cheap enough, they would have undoubtedly gone with 8GB of GDDR5 and taken the small hit to profitability.
I wonder if Sony will shelve VR until the PS5 because they have decided that they need at least 1600p resolution screens to make the image palatable (480p maybe?) and the PS4 is just not beefy enough to run games at 60fps in that resolution, even if they use simplified graphics.
In any case, I hope the Oculus VR will gets us a next gen console generation in 5 years or less. 8 years is too damn long.