The current, most credible rumors suggest that the Second Generation Maxwell will be entirely on 28nm as 20nm SoC at TSMC is mostly suited for Mobile processors as they trialed/tested a 20nm HP and where wasn't any benefits over 20nm SoC and 20nm late this year or early next year is too expensive for very little gains. In short we'll likely see a 28nm GM204/206 late this year and very early next year with a 28nm GM200 soon afterwards. I know some think that we'll see 20nm GPUs late next year but it'll be better to just push 16nmFF (that is basically 20nm with FinFets) Pascal out of the door as that'll be basically likely be modified Maxwell with 3D (not stacked) memory. I bet the memory bit bus width will likely be the same (GM200 with 384-bit bus, GM204 with 256-bit bus) since it is on 28nm.
Die shrinks do not have much performance improvements anymore but if they can have the same performance per watt increases as with GK107>GM107 and able to control the die size then we'll see a big improvement that can easily be. Just do not expect a single Second Generation Maxwell to be able to play 4K with the settings turned up (GM204 will probably be suitable for 1080p gaming, GM200 for 2560x1440 gaming) for games coming out. We've got games like the Witcher 3, Akrham Knight, AC Unity which will probably even make a 780 ti strained at 2560x1440 barring settings being turned down as game developers are using current-gen hardware over 8+ year old hardware.