"Aren't decisions about clock speed for a given ARM reference design taken largely by the fabricators?"
Not sure, I don't think you can just take a design, choose a fab scale and think you can speed up the clock because everything is closer together
Increasing the clock speed potentially brings in timing problems so I would have thought each design would have a maximum clock speed.
I know IBM had to change designs to get their PowerPC chips to a higher Ghz it was not just a case of changing Fab. Although puting things closer together helps.