Thanks for the replies. I'm still dazzled by the dps decrease, and trying to find the reason. Perhaps the rotation I'm using isn't viable anymore for whatever reason, although this seems unlikely. Doesn't make the matter less anything, though.
I believe you can attribute that to the melee AP buff. Melee DPS being higher makes fights shorter, and Shadow Priests are pretty awesome at long fights so cutting time imo is cutting your DPS. It also depends on what kind of gear you're leaving Firelands with, to my knowledge our 4piece was the only one that was nerfed (I can be wrong) and if your doing DS in normal FL gear you have to figure that your biggest nuke took a 10% decrease in dmg while melee took a 10% increase in AP. Also, if I am correct about only shadow's 4piece T12 being nerfed, you would have to take into account that technically you are starting DS gimped.