With the currently available hardware, there is no possible way difficulty adjustments could be 20% for 6 months.  At that rate, even BitFury, MegabigPower, and CoinTerra would not be able to operate their gear at a profit.  Plus, at that rate they'd have to sell more and more BTC to cover operating/electricity costs, causing BTC price to tank, and making even more hardware unprofitable.  The difficulty is self adjusting, and if it got out of control like that, it could go down.

Now, when <20nm ASICS start joining the network at >5GH / Watt, things could accelerate again.