Easy way to find out:
Keep adding spell speed until you're able to do something different, and try to think over what further changes might come with even more spell speed. If it's not near a particular spell speed plateau, it's going to be worth a bit less than more direct damage increases.

That said, keep in mind some safety margin for clipping (e.g. via Sharp Cast or increased reaction times due to panic-mode). Determination will still scale a bit less than either Speed or Crit until at... levels likely unreachable in this expansion... though it's far from a shit stat. The +CritDamage component of the adjusted Critical Strike stat keeps it from tapering off nearly as much as before, and .01 seconds per 25 SS or whatever it the ratio was will be worth a greater % of your remaining GCD the further you increase the stat /decrease the GCD.