I think you're exaggerating the actual impact of the system the platform is running on. The performance difference is definitely there, but it's more to do with the way the games are coded and optimised for a single platform. On top of that, you have developers who stick to certain standards, unlike on PC, which is an open platform.
Besides, the performance difference isn't that dissimilar once you take into account the added benefits of a unified architecture and the actual settings the console games run at.
I'd argue its a case of both working together. But as nobody has a true direct replica of the xbox hardware with which to put windows 8 and on test, its untestable exactly how much difference the Xbox OS makes compared to Windows 8 retail.