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.