I see this thrown around, and yes, it is an advantage, but you're not tapping performance magically out of nowhere. Let's put it like this. To make things simple, I'll use FLOPs (lol) as a performance guage. Let's say PS4/720 have a 1TFLOP GPU (again, for simplicity), while the PC we are comparing them with also has a 1TFLOP GPU. On the PS4/720, you're probably tapping out 90% of that 1TFLOP GPU. On the PC, you're probably tapping out 60%-70% of that GPU. Get it? The PS4/720 aren't going to be working at 2TFLOPs just because it's a closed system. There is still a limit. I don't mean to downplay the situation, but facts seem pretty scarce in this thread.
It's been heavily rumored that PS4/720 are going to be 1.8/1.1-1.5 TFLOP GPUs respectively, both from AMD. Unless you also want to discredit hardware rumors about Wii U as well, then I suggest you take those rumors as equally as you take the Wii U rumors.