I don't think Haskell training per se would ever work.

Haskell is beautiful because smart people find it to be a really good way to express their problems and solutions. Haskell is the way their thinking comes out. So if you're in that 3% you probably know and possibly even use Haskell already because you have that sort of train of thought. If you're in the remaining 97%, you could only do much less or barely nothing at all until you had had a long series of "aha!"s, not often even related to Haskell itself.

Instead of language training, effectively a mindset-changing smartness training is what would have to come first.