Indeed I welcome the option as well. If it doesn't make the runtime noticably slower for non concurrent programs, of course, but I'm confident the authors will make all this a no-op in that case.
Performance backwards compatibility has been a key goal in the Multicore OCaml project. Currently, the overhead for running legacy sequential OCaml programs on the multicore GC is a few percentage points on average. The overhead is low enough that we may not need to provide a "sequential-only" compilation flag.
