cointerra has been quiet lately it seems
Concur. I see them as being the most likely, followed by HashFast.
The idea of 250W TDP in a single die just seems like the wrong approach to me.
Well KNC got it right (came in significantly under simulation) so it is more than just Avalon.
Hyptohetically there is a miss and it does use 450W vs 250W at nominal voltage and clock. Power is all relative. It would be 450W at a given clockrate and voltage not at all voltage and clock speeds. Lower the voltage and power drops by the square. Lower the clock and power drops linearly. If the system absolutely can't use more than 250W there is some combination of clockrate and voltage that gets you there.
This is exactly what Bitfury did.
Well once again it is no different than if a small chip misses. Bitfury did and used more chips to deliver the same hashpower. Obviously they would have rather not missed and made an even larger profit but they adapted and met their promises.
If you miss you need to clock/volt down the chip and use more chips. 500 GH/s chip or 5 GH/s chip you still need to adjust.
... or they could waste precious time and do a respin.
(even if it's a million USD worth of them) 
