Right, I think the argument should be that transitioning from a synchronous to asynchronous programming model can improve the performance of a previously CPU/Memory-bound system so that it saturates the IO interface.

If the system is CPU-bound doing useful work, that's not the case. Async shines when there are a lot of "tasks" that are not doing useful work, because they are waiting (e.g. on I/O). Waiting threads waste resources. That's what async greatly improves.