Multicore benchmarks are on github and are used to drive the changes. When used properly you can see large speedups without affecting much the speed of single core OCaml (which is quite fast).
If you want more details there are monthly updates now on the discuss.ocaml.org forum (just search for multicore). We usually post them also on HN when there is some meat.
Even though rust (or Julia if we talk about numerics) can clearly be many times faster, it is an imperative language with a functional feeling (don't take it in the wrong way, I think both are great languages for different reasons). For less performance critical applications I think OCaml is still worth, it is reasonably fast for a GCed language, has a great type system and the compiler is egregiously fast.
Multicore makes quite a difference in performances for certain numerical code, however it will be hard (if even possible at all) to reach the speed of fine-tuned Julia, C/C++, Fortran or Rust (which does not yet have a proper numerical framework though, afaik). The approach of owl has been to introduce an engine (wol-symbolic) that compiles computation graphs to ONNX format, so that they can be executed on GPU, FPGA, different engines, releasing from it the burden of supporting those various backends.
Personally I find the ease of maintenance and refactoring with reasonable speed a good compromise, I think its sweet spot is for exploratory code that does not require bare metal speed and yet should be fast enough.