A card like this is already available and it is called 680mx.
Just overclocked to 770mhz from original 720mhz of the 680mx.
Not quite. First the mx is only available in certain iMac's. It has a 5 GHz memory clock as opposed to the rumored 6 GHz clock on the 780m. And don't underestimate the effectiveness of GPU boost 2.0, it's much better at sustaining boost clocks than the original. And this isn't mentioned anywhere in the article, but I wouldn't be surprised if the 780m uses the upcoming gk104 revision that will probably show up first in the GTX770. From what I've heard it should provide slightly improved power efficiency and thermals over current silicon.