If you have a GC and no static typing, is it better than other functional languages (many of which have benefits of both GC and static typing)? Not a rhetorical question, I have never used a GC with C.
If I were to guess, the C version would have a bit better performance in a single-threaded context due to the fact that I would occasionally use tricks to avoid reallocations/frees, and the Boehm GC is opt-in, so when I was reasonably certain I could handle the memory correctly, I would do it myself, minimizing how much was actually being done by the GC.
I do feel that a static functional language like Haskell might perform well (maybe even better-on-the-average-case?) in a multi-threading context, since I personally find dealing with locks in C (and C++) to be very difficult to do correctly, so utilization of some of the cool tricks in Haskell to avoid manual locking might benefit. I was too much of a coward to use threads and locks much at that job, so I haven't had much of a chance to test it.