Assuming they do start to deliver, the first recipients will be the only ones that have any chance to pay off their device. Soon after delivery they will not be able to mine more than the best GPU can today.
What are you basing that on?
Please remember that as diff goes up, so will the value of BTC, if it doesn't then no one will mine.
Who is going to run a 50gh unit when the diff is 300m if BTC is $140? No one that is who!
But if BTC is $1000 and you earn 1/10th of a BTC a day, that doesn't suck.