Tyrian

We used the exact same setup as you (3 paladin, 3 shaman, 3 COH priest, 1 druid) although I would prefer to take an 11th healer (extra shaman or druid) if we had available. Either is fine.

The key turnaround point for us (after raid healing failures) was very specific assignments. 22 people in the raid either have a in-group healer directly responsible for them - or someone else assigned to directly watch them and the other 3 MT's have their own personal healers. There is no 'ok healers, just heal whoever and try to keep the raid alive' - all raid healers have ~4-5 specific people who they are wholly responsible for keeping alive. You must take careful note of which groups are physically 'split' between top/bottom camps and assign the correct type of raid-healing accordingly (COH priest or Resto Shaman or Lifebloom)

For example, if you have a

Feral Druid
Hunter
Hunter
Resto Shaman
Holy Paladin

You would need to make sure that your COH priest does NOT cast coh directed over the feral druid, as him being down the bottom will mean it not have enough range to hit the other 4 people up the top camp, and your group members will start to die. Same with a shaman, chain heal bounces must be directed at the 4 people up top. But COH priest or Shaman is a good option for raid-healing this group. The Feral druid MT obviously has his own healers - and doesnt require the raid healers attention at all.

Another similar situation is the Warlock MT group. Your warlock MT (who stands down the bottom) is possibly in a group with at least 1-2 healers (who stand up the top) because hes a waste of a spot in a DPS/spriest group - and the group is split. If your Warlock MT group looks like this:

Warlock MT (Lets say hes affliction spec, with an imp out for your warriors)
Warrior MT
Warrior MT
Resto Druid
Elemental Shaman (Dropping WF, acting as a agro dump)

Theres only two people in that group that need a raid healer assign, since the 3 MT's have their own personal healers: the resto druid and Elemental Shaman need someone to raid-heal them.

I would get the druid to lifebloom the first four (3 tanks + himself), and one of the resto shamans in another group can simply watch the elemental shaman in addition to their own group. This other Resto Shaman would ideally be one whos already healing a group that only has 4 other members up-top, like the FeralDruid+hunter group - for a total of 5 people to watch only. A COH priest would be a terrible option for servicing this Warlock MT group. You must be either using a lifebloom stack and/or Chain Heal for the 2 non-tank members. Its not worth assigning one full-time shaman just to heal 2 people either, so splitting it up and having 2 healers from 2 other groups watch 1 person each from this group - makes alot more sense.

In all honesty, we had some attempts with 11 healers (4 shamans, 4 COH priests, 3 paladins) that were a healing disaster and terrible compared to other attempts where we used the 'inferior' healing setup of 10 healers (3 shaman, 3 COH priests, 3 paladin, 1 druid). Why? Its solely because of the way we assigned Chain heal and COH to the raid. Bringing more raid healers in no way guarantees you will have better raid healing. You might even be better off going with fewer healers but cleaning up and adjusting your healing assignments.

Thats probably the best lesson I learnt with regards to healing setup for our time spent on Twins.