Honestly, I think home miners should only mine as a hedge against bitcoin price stagnation, which could theoretically lead to difficulty stagnation.  If you are long on bitcoin, have a sort of 15% mining operation.

Lets say you buy 10 BTC.  Maybe it would behoove you to buy 2 Antminer S3's.  If the difficulty skyrockets beyond profitability too quickly, then it would be probably safe to assume that the bitcoin price would have reflected upward momentum to account for the increased difficulty.

Again this is in a perfect world and we all know miners aren't rational.  But in the event of a +20% btc price, you would see a lot of old miners turn back on, and other companies roll out batches to gain hashpower.  At this point I can see maybe the difficulty doubling at the current BTC price.... anything beyond that and I'd start to worry.