So what would a Bitfury28 look like. With a die shrink we are looking at (55/28)^2 = 4x transistor density. Lets be generous and say 50% higher clocks are possible so maybe ~6x die efficiency going from 55nm to 28nm. That still puts a purely on paper theoretical Bitfury28 die shrink in the ballpark of say 0.7 GH/mm2 (nominal), 1.26 (overclocked). Of course this would just be a die shrink with no architecture improvement. There are obviously power issues which prevent Bitfury55 from acheiving the 5 GH/s (420 Mhz) design spec maybe if they were solved it would have higher die efficiency.
since BitFury's aim was 5 GH, and he achieved approx half that... i think its likely that whatever errors were in the design that prevented it from running at full rate, would get fixed next time around... so, with the die shrink from 55nm to 28nm (4x density of hash cores), and the errors corrected (2x speed), and perhaps some architectural improvements (e.g., fix the daisy chaining which makes it unreliable and hard to scale properly) ... i would've expected the BitFury28 chip to be at least 8x the current chip's performance... i.e.: 16 GH (or more), in something thats probably pin compatible with the old one in a low cost qfp as before. Since its unlikely to run at an 1/8th of the power of the previous one... I'm presuming that the next bitfury28 design will need heatsinks on each chip because the likely power consumption might be in the 8 watt ballpark.