GThe DPS companions are about as mediocre at DPS as the tank companions are at tanking, and probably not as good as even a lousy player in most cases (and that's saying a lot, given the lousiness of some PUG DPS).
True story. Did The Foundry first time with 2 of my friends (me as sniper, friends were assassin tank and operative healer). Got stuck to HK-47, tried to kill it for a long time with a dps comp (nicely geared Vector), but killed every time. Then we recruited some random dps to our team and after 1 wipe (it was the first time to our new dps) istantly got it done.