... what happens to Bitcoin when mining ROI suddenly takes a 50% hit.
ASICs will eventually make mining marginally profitable. ROI will be in the 1 yr+ range.
The good news (if you can call it good) is that profit margins of h/w manufactures will be razor thin. More competition, better customer service. No more Yifus to deal with.
Eventually, the profit margins for both mining and h/w makers will go down to big fat ZERO and this will be the end of bitcoin.
Unless entities (private foundations, banks, governments, h/w manufacturers) adopt bitcoin and protect it via "at loss" mining operations.
One thing is for sure, without mining there will be no bitcoin.
BTW, OP is right on the money. Maybe a little conservative, but right on the money. Bitfury will add thousands of TH/s