Well I understand your point, but you are mistaken about something, the gpu will run close to that number more often then not. Here is why:
When we for example run our game the cpu is most often not utilised at 100% load, that’s due to the different workloads in the engine and the capped frame rate in consoles and the like. That means even if you have hardware overhead, the game will not make use of it. This is the correct approach as you want to make sure that throughout your game the frame rate remains constant, even with heavier scenes, which is why a little overhead (a small number) is always fine. In fact this results in some idle power for the cpu. Now if used correctly Sony devs could simply use the additional power for the gpu making it run a little faster. But again same principle goes for the gpu, you constantly want to hit your fps target and to achieve that you have to assume a constant power target from your gpu. This is where this feature must work, otherwise you would see a lot of variable frame rates, which you as a game developer not really want to see. So if Sony could not provide a relatively speaking constant power target, they would not have implemented this feature. I believe it will work quite well. But again this will not migrate the power difference between the two consoles, Xbox is more powerful but not to the extent that some people believe. Instead, this in my opinion is the best generation we have ever had for game devs as both consoles provide everything you want from a next gen console.
Cheers.