I replaced the array/"surrounding" list with an unboxed vector, and removed the strictness annotations and it runs at ~0.85 on my machine, while the C code runs at around ~0.5.

The code is so incredibly close to your SML code (where you also spell out that you are using the Vector type). I am just blown away by how matter of fact you were in your statement.

Please either change your benchmark to use the same data type as all of the other examples, or stop using it as an example of Haskell's performance.