In my opinion the specs for a 20nm GPU like that are perfectly believable whereas the release date isn't. Surely if this were to launch it would be in early 2014 at the earliest?
Surely with it being a newer architecture the shaders would be more efficient than the old ones, therefor the actual floating point power of the card would be higher than mentioned?
Also I don't see them using a 512Bit bus, that's added complication and manufacturing costs, besides the fact that a 5GHz ram speed seems a little on the low side compared to what we've seen from more recent graphics cards.