Not that it is of any importance but your 28 vs. 20nm point is a bit weird. Whether real or not the config for that hypothetical GM204 suggest 7.9b transistors; considering the TDP is set at 230W and even a GTX780Ti with 7.1b transistors under 28HP is at 250W, there's nothing absurd considering that, at least not from a few miles distance I'd like to take from that table
Ailuros, my point is that this card is supposedly replacing GK104, not GK110. Yes, GTX770 has 230W TDP but that it's already pushing GK104 a lot. If they do the same to GM204, on 20nm, what does that tell us about Maxwell perf/watt? I know we still have to know the chip's performance, but why would they push the TDP so hard from the beginning? It all doesn't make much sense to me, unless Maxwell is another Fermi type event...
I know you are giving relevance to GDDR5 power consumption, but as I think we have seen with gm107, the larger L2 cache has helped the chip to overcome somewhat the 128 bit memory bus and low speed memory. GM107 can approach GTX650 Ti Boost performance levels with 60% of the latter memory bandwidth. Does nVidia really need 7.4 Gbps memory on gm204? A rough extrapolation would put 60% of GK110 memory bandwidth at 200 Gbps, not that for off from GTX770's 224.
3200 cores is only roughly 10% more than GK110 2880. Gm107 packed roughly 66% more cores than gk107, while increasing the die size much less and no big increase in TDP. Ignoring for the moment the conspiracy theories about no interconnect on gm107, why couldn't they do the same for another Maxwell chip on 28nm?
GK107 - 1.3B Transistors in 118mm2
GM107 - 1.87B Transistors in 146mm2
So, 43% more Transistors in 23% larger die size, or a 0.53% increase in die size per each 1% increase in Transistors number.
GK110 - 7.1B Transistors in 561mm2.
GM204 (28nm) would have roughly more 11% more Transistors, so die size would be 5.83% larger at 593 mm2. It is quite large, but this does not include further optimizations nVIDIA could eventually do to pack more Transistors, plus we are talking about a 256 bit memory controller, versus GK110 384 bit.
In short, I still think this chip could, maybe, be doable in 28nm, justifying the 230W TDP. Additionally, IF these specs are real, normally they leak not so far from the chip, launch, say 3 months. Would it be reasonable that a 20nm reasonably large chip to be coming in June/July?