Isn't your productivity gain coming from using a statically typed language rather than Haskell specifically? It feels like any statically typed language is a leg up from Python :)
I think it's a bit more than that, Haskell has a much more sophisticated type system than quite a few of the popular statically typed langauges ( C, Java, C# ). Static typing is only the start, to be really useful we need modern type inference methods and a concise way of expressing generics. Both Scala and Haskell do this very well.
In this sense I suspect the OP would say the same about any ML-like static type system, but not many other static type systems.
but to me Haskell is a different league.. Haskell-typing actually helps /me/, where i feel C++/Java-typing merely help the compiler.
It's not a profound part of writing code in Python for anybody that's spent more than a few weeks with it.