I see no reason why act 3 mercs shouldn't have drastically buffed damage, namely given cold/fire/light mastery (and buffed inferno for sorcs and mercs alike)
there's really no remotely practical use for their damage output, even the best possible mass facet griffs/nightwing and ormus on cold/light mercs still sucks, the lightning barely gets relevant damage that's still eclipsed by other mercs, and yet it only casts lightning 1/3 of the time, it casts useless charged bolts 2/3 of the time
the only uses right now for a3 mercs are as mules, or the absurd mass poison death facet on a level 49 merc for low respawn costs / 10 hp in hell