I don't know of a comprehensive study that has been done (although there probably is) that links qualified individuals immigrating with net economic growth. However, if you look at it like VC investing, it would be worth letting 1000 people in to get an entrepreneur capable of making a Stripe, Tesla, etc. According to this forbes article[0] 40 percent of Fortune 500 companies were started by immigrants. Obviously, many were started a long time ago, however I think the sentiment is the same. We want to benefit from brain drains in other countries, and we want smart people who are emigrating to see America as a top choice.
[0]http://www.forbes.com/fdc/welcome_mjx.shtml