Wanted to hang on to this thought:
There is definitely a ceiling to the amount most classes can benefit from a shadow priests' mana regen, since a strong shadow priest can literally return mana as fast or faster than some classes need to spend it. In that case, all a shadow priest is really doing is making life easier and saving them a few coins in mana pots, etc.
Mage is really the only class to my knowledge capable of ratcheting up their mana expenditure in exchange for performance at a level that can easily exceed a strong shadow priest's mana return. Since that is the case (to such a remarkable level), it doesn't seem as though it even matters if shadow priests get nerfed or mage mana regen gets buffed, there will always be a direct and noticeable correlation between what a shadow priest does and a marked improvement of mage performance. Needless to say, as long as Arcane Blast exists, shadow priests will not be out of a job in the raid environment any time very soon (even if bringing a shadow priest means taking one less mage), since a shadow priest might have to return upwards of 10K mana each minute in order to reach the maximum rate at which a mage can dump it.