I suspect there is a point of diminishing returns with this though; the extra complexity in the type system gets you less and less benefit but at a greater cost to code complexity/comprehension. It could explain why the mixed Scala/F# paradigm has more industry usage than Haskell right now. i.e. I am trading a type system that has a little more expressiveness for a large amount of libraries, ecosystem, rock-solid VM, testing, corporate buy-in, etc. In a .NET shop for example its much easier to just drop an F# DLL into your codebase (in VS create a new F# project in a few clicks); there's no need to change anything else (e.g. build machines, build tooling, testing tools) since it's all IL anyway; same goes with Scala and Java.

My experience in coding functionally has shown that mutation (especially if localized and doesn't leak outside of a function) can really help functional code become a lot faster. From what you describe C# is slowly becoming an F# clone; which may be a good thing or not. Still think that if you need these features you should move to Scala/F#/Ocaml/Haskell though since I suspect these features will be added to C# in a clumsy/compromise way given its roots (e.g Scala and F# already have exhaustive pattern matching, async streams/iterators, better type inference, etc.).