Demonology is terrible spec for primordius because minions(wild imps and doomguard) only do 25% of the damage to the boss. And to be honest your strat sounds kinda weird, like destro lock finishing blobs before they reach boss. What we do and most of the guilds(i think) assign few players to the black blobs and ~4 multidotters(SPs and affli locks) to killing blobs whole fight.
So basically affliction for killings blobs(it's also OK spec for tunneling boss) and destro for boss(RoF and FnB conflg for embers). Demonology is by far worst since wild imps does most of your damage and now they do only 25% and it's not so good killing blobs.