With our setup we have been running 7 healers (1 Paladin, 1 Shaman, 1 Druid, 2 Disc Priests, 2 Holy Priests) and have been having the Paladin, Shaman, Druid, and 1 Holy Priest take every portal.
Last week we had no problems with this encounter, but this week was a bit laggy until we have everyone destroy their Lovely Charm Collector's Kits after which we killed it in 1 pull. Prior to that we were hitting around 7:30 at 80% and getting overrun when we had 5 healers taking portals and just leaving the 2 Disc Priests out to raid heal. With the 3rd healer centered outside we gained a much more stable composition and the healers were having no problem building 25+ stacks due to less competition (having everyone meet up before circling didn't work out for our healers, as one late person slowed down the whole group considerably).
Bloodlusting later in the fight is definitely better because healing with 30+ stacks will always be better than healing with 20+ stacks, and it also benefits the DPS outside for controlling the ever increasing add spawns. On our kill it was very clean, and a Earthbind Totem/Frost Trap at each of the 4 gate entrances considerably increased our healing output by never allowing any Suppressors any healing debuff channeling time.