I'll give you rigorous, but how is it proven? I think it is a fair guess that the number of non-tiny programs (>100KLOC) making use of the "expert" concepts and skills is ~0. The number of large programs (>1MLOC) at the proficient level is also ~0.
(This is intended as a serious question, not a troll. It often seems that in discussions of FP, and particularly of Haskell, someone will suggest that there are few widely-known, large-scale projects written in this style that can be used to evaluate its effectiveness, and someone else will then reply with one or more of the same very small collection of larger projects or high profile organisations using FP/Haskell that are publicly known.)
Please don't talk about what you don't know. Standard Chartered's Haskell compiler is written completely from scratch and is not based on any existing compiler.
(Also, while it has little to do with my point, I've also heard that the person behind SC's Haskell compiler is the one who'd written the first ever Haskell compiler, long before he started working for SC; is that true?)
[1]: Please don't take this too literally. In the two decades that have passed since Haskell was declared the language to end world hunger, someone may have written a bigger program. Maybe even two.