It doesn't matter, while there is not currently Mantle on PC, there is no low level access API, console gpu's are significantly more efficient due to being able to code so much more specifically/efficiently for the hardware.
The last gen had gpu's, for the xbox, similar in actual power to a x1900 but closer to 290xt in architecture, the PS3 had something akin to a 7900 gt in it, but neither would perform close to either card, both would perform significantly, massively really faster.
Mantle should bring performance improves, sometimes very significant but it still won't be on the level consoles achieve for multiple reasons.
I wouldn't be surprised if say a console 7850 equated to a pc 7970, that is the kind of situation we're looking at. You really can't make a direct comparison, even less so when the available power on the PC means dev's spend less time optimising. Raw power will be similar 7870 vs 7870, throw in low level API and specific coding for it, and you're closer to 7870 vs 7970, then add in the can't be bothered to optimise, and it may get even worse.
However AMD/Nvidia made some really high bandwidth, high res capable cards, that consoles would fall well behind in.