It doesn't seemed to get cited as much as it should, but Ryan Trinkle's CUFP presentation in 2013 is a great success story. The executive summary is that they rewrote a 43K line Groovy app in Haskell. It resulted in an 80% code reduction, ~64x performance improvement, and completely eliminated a number of major bugs with the Groovy system. Here's a link directly to the results portion of the talk:
https://www.youtube.com/watch?v=BveDrw9CwEg&feature=youtu.be&t=1208