I think your dps is a little low for 540 ilvl (do they have capes?), it depends on specs and itemisation too but I think most people should be around the 200k mark as a minimum in that gear, it may drop a bit in p2 if people get targetted by fire (melee is prone to this). I currently raid with two groups, one with lower dps gear (around your level but min ~175k at the end of the fight) which 3 heals it, the other is quite good (dps 220k+ I don't really keep track though) and we 2 heal that.
If your healers aren't geared or skilled I would probably recommend 3 healing it, what potential healers do you have btw?
Edit: Druid and Shaman are probably the best two healers for p2, if they are decent I would recommend two healing it. For HPS reference I noticed I (resto druid) was doing ~110k hps while on the ground in p1 and maybe 120k~ after p2 is over, make sure your druid is popping his mushrooms in p2. Our disc priest (who goes up towers) was maybe 80k in p1 and spiking to ~100k after p2 is finished.