Do you have a source for that?
Most rumors I have read point to GTX660 series being based on GK104, which will have 1152 or 1344 CUDA cores (with lower clocks), along with 1.5 GB of GDDR5 memory.
http://videocardz.com/33859/nvidia-geforce-gtx-660-ti-to-be-released-on-august-16th
Roadmaps rarely get pushed forward. If anything they get delayed. If AMD has HD8000 series for 2013, that's probably the best case scenario. Considering HD7990 and GE cards aren't even out yet.
20nm isn't going to be ready by mid-2013. 28nm node has just been in production for 6 months on the mainstream level. Look how difficult the move to 28nm has been. You think 20nm will be easier? If anything, it'll be even harder. I am pretty sure Maxwell and HD9000 series will use 20nm but that's 2 years away. Next round will be built on 28nm.