How much is hard to port a haskell program to C ? If it will become harder and harder, (i.e. for parallelizations) than it's fair to choose haskell for performance, but if it's not, I think it's hard to think that such a high level language could ever compile down to something running faster than its port to C.
There is a logic programming language called Mercury; it has strict polymorphic types and strict modes and it supports functional syntax as well as Horn clause syntax. You could think of it as 'strict Clean with unification'. In the early days, they had a list processing benchmark where the idiomatic Mercury version of the program was faster than the idiomatic C version of the program, despite the fact that at the time Mercury was compiling via C. The answer was that the kind of C code being generated by Mercury was not the kind of code any sane programmer would ever have written by hand. It really does depend on how you write it.
Will hardware really go for hundreds of cores ?
You can already buy a >700 core machine (I have _no_ idea how many chips are involved in that) for Java.