YJ- we are using a single tank (Thith is now Cataclsymically Superior) and 6 DPS, but are still short. I think we have the mechanical quirks ironed out, we have the execution, and now we need to hit heroics and theorycrafting boards to bring up the performance.
Pro tip - A tank in his mouth can DPS the exposed head even before it's chained to the spike. I was taking full advantage of this to help keep myself up with death strikes. I wonder in Syrennia in Jagger is sundering the head in preparation for the chains? If not, that would probably be a great practice. I also wonder if ranged can hit the head while the tank is in his mouth - that would give us a minor head start when we have ranged up front (who aren't busy with parasites during the window of opportunity).
The point of making all the ranged take a stint on the outside orignally allowed us to all learn the fight mechanics together, and allows us to mix and match the parasite DPS until we have the correct combination. However, I don't believe that keeping all 4 ranged DPS on the parasites is the final answer. Ideally, we'll want to get to the point where we can have 3-4 DPS on the boss full time and only have 2-3 DPS in in the back handling the lava parasites.
The theory behind keeping all the ranged on the parasites "just in case" is based on the simplified notion that the raid can do a consistant amount of DPS against multiple targets as single targets. That's just not the case. Single targets benefit from cross-buffs (earth and moon, sunder armor, etc), and we need as many of those benefits as possible to push up the raid DPS. Additionally, single target DPS allows the DPSer to get into a good rotation or priority queue. On the other hand, AOE is ineffecient by nature. The need to reposition channeled spells, the tapering off of aoe effeciency as targets die off, swapping targets (removing the ability to spell queue as well), and dots that don't last the full duration will eat into our damage output potential and mana effeciency. The fewer raiders we have who need to worry about this lost potential, the better for the raid output on the whole.
The player who did the best damage on the parasites, while providing valuable slowing, was the hunter we picked up. We should keep her. When we get to the point that she and another backup DPS/CC (moonkin or Xelian are probably the best bet) can handle the worms by themselves, we should have enough DPS to down the boss (assuming all the other DPS scale up approximately evenly).
In Jagger, YJ and Whateley do the pillars with Zivilyn as their personal (and quite mana-starved) healer. They rotate CD's (eg, trap the even numbered waves and use warlock pet's aoe on the odd numbered). We should do something similar - trap the odd numbered waves, starfall on the even numbered waves, and mushroom/hurricane/multishot every time. I think everyone is blowing their CD's on the first wave (which overkills it) and then has problems on the next wave.