I'd had a look at Webstats average dps values for a mage at my level (SSC) and it's 1900, mine is around 800-900 and I thought I was majorly slacking (and as I'm one of the top in my guild that we were vastly underperforming). I feel a bit better knowing we're probably around where we should be.
On-topic: I've never been to those fights, but from the ones I have experienced that are more about brains than big numbers (i.e. phase transitions at Hydross and Leo + inner demons) for phase transtitions showing the target of target should let you see who's not paying attention. I generally just tell everyone to watch their threat during transitions in an ever increasingly unhappy voice, reminding people that we're not going to do the fight if they keep pulling agro and also that things like health/mana pots or buffing cause threat too. If there's a persistant problem with a specific player either an officer or class leader will have a chat with them.