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.