But I doubt it's a reasonable estimate as it would represent about 50x the current hashing power, or add another 27000+ TH.
That's a lot of hardware, even at 400GH/s per unit (≃67K units).
Sure, it's a worst case estimate. But don't forget, difficulty is going to hit 65M today, so only 40x gets us to 2.6B. 100% increase each month gets us to 1B difficulty by end of year. That's not that unreasonable. There's a lot of hardware that's coming online Q4.
At $8 per GH that would be $216,000,000 in hardware. (Monarch & Cointerra)
At $15 per GH that would be $405,000,000 in hardware. (Hashfast)
At $20 per GH that would be $540,000,000 in hardware. (Bitfury October)
At $50 per GH that would be $1,350,000,000 in hardware (BFL, AsicMiner, Avalon, Bitfury August).
Possible but unlikely.
Note that manufacturers likely have to book a certain amount of chips (I heard sets of 25 wafers) at the factory. So the number of chips actually hashing in the future might be larger than the number of orders that the manufacturer actually manages to land. (Also, no KNC??)
Difficulty will overshoot by a lot into deeply unprofitable territory before the flow of chips reduces.