Spell speed seems to have been tested in the future when we're all in iLvl 300 gear, because at the moment it requires downright massive amounts to be anywhere near useful. I'm at 450 SS, and my GCD is 2.39s. There's a scale obviously, but just taking the average into consideration, that's approximately 0.01s per 41 SS. That's incredibly inefficient. Additionally, SS doesn't effect HoT tick rate, so it's not like Druid on WoW where you can squeeze out an extra Regen tick with enough SS. Finally, take into consideration that you are rarely spamming spells. As a WHM, you should be playing proactively, not reactively; precasting spells to land at the moment your tank is hit by something (Death Sentence, Table Flip, etc), which means that SS isn't actually being utilized.
It's possible that it'll be useful in the future, but at the current moment, DET > SS.