OCaml has cool stuff like MirageOS, as one compelling lib (still a research project). It's a library that constructs unikernels. You write your application, build it, and out comes a VM that you can run.

I feel that many of the advantages of switching to OCaml are minimised when you are already using an ML!

Soon, there should be a compelling performance case for using OCaml. There are rumours that in the next release, there'll be an MLton type whole program optimisation setup, which should reduce the cost of abstraction to zero.