I think that Fil-C, unlike normal C allocators, could use a copying GC instead of a tracing GC, because it isn't exposing the machine pointers to user code. But I don't know if it does. Copying GCs often suffer from high memory overhead, but for example OCaml's GC is pretty frugal with memory, and RC by itself doesn't guarantee good worst-case latency—decrementing a reference to the root of an arbitrarily large tree can have arbitrarily bad latency in RC. You probably already know all of this, but someone else reading the thread may not.

So I'm not sure there isn't a single winner between ARC and GC, but you could be right.