“A simplified version of real-world problem was chosen by the Naval Surface Warfare Center (NSWC). This problem, a geometric region server (geo-server) is one component of a much larger system, NSWC’s AEGIS Weapons System (AWS), which NSWC is in the process of redesigning.”
“The participants, each considered an expert programmer in one of the programming languages being tested, was asked to write a fully functional prototype of the geo-server, while keeping track of software development metrics such as development time and lines of code and documentation.”
“The results indicate that the Haskell prototype took significantly less time to develop and was considerably more concise and easier to understand than the corresponding prototypes written in several different imperative languages, including Ada and C++.”
[1]: http://www.cs.yale.edu/publications/techreports/tr1049.pdf
I think it's imperative to question the "reliability and performance were dramatically improved" kinds of statements with quantitative research. It's just that this kind of research is insanely hard.
(Disclosure: I'm definitely on the FP team, but I want do be able to argue objectively and quantitatively for the benefits of my approach.)