The logistical issue is that GK110 is generally massive to the point ANY kind of chip on 20nm won't be possible to be made small. I suspect they will go to a medium/large chip at first, something like the 290 or slightly smaller.
They will have at least 6 months to refine the process. The chips will never be cheap anyway if they are big.
Whatever the size of mobile/tablet chips, they are complex.