Those very smart developers would be just as effective in a mainstream language, and probably more so in an appropriately chosen mix of languages. It seems like the real advantage of lisp is that it self-selects for smart people who program for fun, and these people typically are also good developers.
... if you are smart enough. If it were that easy to get more productivity, everyone would be using Lisp. But you need to hire very smart developers to get that prodictivity, and most developers are by definition average. Your average developer will be frustrated, not more productive, with Lisp.