In the future with new nodes getting more and more expensive, the focus shifts on architecture.
A 28nm Maxwell GM104 (or similarly named) could take the performance position of the GTX 780, but with a much smaller die size and a little lower power consumption. It would compete better with AMDs new GPUs that are bound to come out some time in spring, tool Or do you think they will stick to Tahiti and Pitcairn until 20nm?
Anyway, right now the signs clearly point to an early Maxwell release:
Editors of hardwarecanucks and videocardz.com, informed users on XS forums are all in agreement that samples were done in summer this year. This makes a spring launch very much possible. GK104 will be 2 years old by then and ripe for succession.
And last but not least, don't forget the 28HPM process. It's quite new and as far as I have read, it offered very significant gains to some Qualcomm SoCs vs 28HP. 28HPM is TMSC's best 28nm process with improved perf/W characteristics. Additionally, you can select a broader range of threshold voltages, in essence fine tuning all the transistors in your GPU (you have slow transistors which use little power and fast transistors which use lots of power). If you're power limited, 28HPMs gain over 28HP is almost as much as the gain from 20nm TSMC, or so I was told. This mainly applies to SoCs like in smart phones, but there was a rumor going around that the XB1 SoC uses the 28HPM process.