From observations, you don't gain much beyond 650, it ends up around 14.5-15kh/s regardless. There is some other limit in the system which prevents it from doing more, may CPU/RAM limitation.
That has been my experience as well. I'm currently running 656 but there is no difference from running at 646.
In the morning I'm going to put a current probe on my Z9 and plot power over time and relate it to clock setting, to see if it even uses more power at higher clocks.