At worst, you'll have to explicitly wait for all your async calls to have completed and produce results. I suppose normally the implicit wait occurs on attempt to access the result. I also suppose you have to do it anyway in a multi-processor system, and you can't be doing it on a single core.
At best, some of your CPUs would be able to run calculations for two especially fast-to-compute pieces of volume space, while some other CPUs would be busy computing a particularly gnarly block of the volume space.
The problem is that async adds a lot of overhead, and while the mythical sufficiently good compiler could remove it, in practice it is a lot of work for little benefit.
For those CPU bound jobs that do not fit in the classical openmp style scheduling, more dynamic async style scheduling might be appropriate (cilk style work stealing for example) but the async granularity is hardly ever the function boundary.
At best, some of your CPUs would be able to run calculations for two especially fast-to-compute pieces of volume space, while some other CPUs would be busy computing a particularly gnarly block of the volume space.