I see what you did there 
I think he is referring to this paper (which does contain the lowering aggregate US growth by 50% claim):
http://faculty.chicagobooth.edu/chang-tai.hsieh/research/growth.pdf
Basic argument if I’ve understood it correctly - the places that generate money are expensive places to live and have restrictive rules on whether new housing is allowed. This means that poor people find it hard to live there. Which means that the productive city has fewer workers available to it, than it would have otherwise, which means it produces less than it could.
Basically, the potential workers all live where there are no jobs, because they can’t afford to live where the jobs are. If there were a way to get the potential workers where the jobs are, the country would be generally better off.
I have to admit I don’t follow their formulae at all but they certainly seem to think they can prove it.
I gather this chap is famous/notorious in the US? He certainly seems able to take a good point and bury it thoroughly.
