The 12 nm FFN process should be around 20% more dense than 16 nm FF+, that current GP10x GPUs are made on.
Also, you accounted the die sizes, but not accounted for changed layout of the architectures, different memory controller counts, etc. IMO GV104 will not have over 400mm2.
Also I do not believe that for GV104 to be 30% faster than GTX 1080 Ti is required more than 3072 CC's that are using Volta architecture.
20% more Cores, with 50% more IPC, thanks to shift from 128Core/256 Register File Size, to 64 core/256 RFS, gets you around 65-70% increase in performance, at the same clock, as GTX 1080 had.
Only problem you get is that the throughput of the cores is so big that you have to have enough memory bandwith and very fine grained scheduling to feed the cores, all of the time.
I think xpea some time ago has written that Volta will have Tensor cores in some way/ shape or form on consumer GPUs, for compatibility reasons, which may suggest that we will actually see true Volta architecture, instead of reused, repurposed GP100 architecture, which also would have same 64core/256 RFS, but not that good scheduling, as Volta has(which is very important, also since you have such high throughput of the cores...).
Next topic: GDDR5 prices. Currently 20 nm GDDR5 prices went up by 30% to around 6$ price tag per chip. This makes GP107 memory subsystem to cost Nvidia not 18$ like it was before, but 24. That is 6$ increase, and we can expect that this GPU price will go up by 10$, in upcoming months.
However, Micron and SK Hynix are going to manufacture GDDR5 16 nm in 2018. What this means is that prices for this memory will go down by around 50%, to 3$. For GV107, most economically affordable, and logical move will be running 192 Bit memory bus, because GDDR6 memory will cost around 10$, for each chip. 40$ for 256 GB/s memory bandwidth, vs 18$ for 192 GB/s? Its way easier to get to 149$ price tag with 18$ memory subsytem rather than 40$ .
Also consider this: GV104 will use GDDR6, on a 256 bit memory bus. That makes it 8 chips, for around 10$, each.
Well, this GPU will not be cheap. I expect the same rollout as for Pascal was: first GV104 - GTX 2080 costing 699$.
Those are just my thoughts on what might happen.