As you mentioned, performance is more dependent on bandwidth and latency than capacity. 8GB is fine.
I don't understand more memory is needed here or MHz for max performance?
It's not necessarily a linear solution. Since you're using an APU I would recommend 8GiB as being a healthy amount of memory. Go with 16GiB if you can afford it, but not if it would mean sacrificing IO bandwidth.
16GiB of DDR3-1866 if possible, but 8GiB of DDR3-1866 if not.