I'm not so sure there will be that many incremental changes made to chips once they start to hit the process limit. I say this as I expect the margins on chips to become so slim that it would be financial ruin to invest in the NRE for another 28nm chip once you already have one.
I guess it all depends on how "good" existing designs are. Taking HF just because they provided die size, hashrate and power info.
One 18x18mm die is able to do 400 GHash (nominal - more overclocked**)
Hashing per square mm:
18x18mm = 324mm^2
400 GHash / 324mm^2 = 1.23 GHash/mm^2
Reported power consumption is 400 GH / 250 W = 1.6 GH/W.
So HF claims 1.23 GH/mm^2 and 1.6 GH/W @ 28nm. It remains to be seen how "good" those are for 28nm. Are they very good, or barely adequate?
If their simulations show an improved version is only marginally better (say 1.4 GH/mm2 and 1.8 GH/W) I agree it wouldn't make sense to try and squeeze out more efficient chips. On the other hand say >2.0 GH/mm^2 and >3 GH/W) are possible. Once we get multiple vendors with stats based on real silicon we can start to get a better idea of relative efficiency.
Just responding to say that I agree on all points. I imagine the first 28nm designs may be more safe than elegant to minimize risk, leaving performance on the table. 2014 will be exciting, 2015 maybe not so much.