Still playing around with it, but my shaman heals me fine against TDS T3 trash mob, so long as it's just one. My tank is pretty poorly geared, no raid gear at all, some T3 visable, mostly T2 TDS stuff. DH HA, my shaman is fine as only healer, so long as I don't pull more than 3 at a time (only tanking all 3 at once long enough for my enchanter to mezz).
I was thinking shamans have comparable heal spells to druids, but looking at the lvl101-105 spells, druids would seem to have much better spells. My shaman is mainly only working with a 2.6sec cast 12k heal and a 2.1sec cast 3 waves of 3500 heal, the rest is for emergency heals. I don't use KISS, so not sure on how to set up that INI, but I notice for the mac that I use, getting my shaman to cast heals faster involved not letting him think about what to cast.
I had my 'reckless' emergency heals set to be used if I drop below 70%, my spam heal 'mending' set for 98% and my AE wave heal 'surge' set at 96%. I am not going to pretend I know what the deal was, but the end result was my shaman stood around not healing a lot when I can plainly see his spells were ready to go. Just dropping my emergency heal to 40% got him to cast his spam heal over and over without much delay as well as occasionally casting surge, which works out great for me. Trying to figure out how to get him to Twinheal+surge via the macro, but still working on it, although in TDS T2 and above, the DPS on my tank is enough that I don't think my warrior can survive waiting on my shaman to twinheal and then heal. I got it set-up where I /tell the shaman and he does it though, but I don't bother because him casting twinheal creates the situation where I need a bigger heal than what his normal heals already provides, assuming my tank survives the wait.
I also set him to not buff, not check anything other than heal related stuff and instead just does all that with a /tell. All he does is cast counterbias slow and then straight to healing, which is just spamming mending and sometimes surge. Adds? He does not debuff adds and just keeps healing. My chanter will take care of adds.
If I want a heal over time, sluggishness, or Growth, I /tell at the appropriate time. I don't see the mac/ini 'deciding' this well enough, so better off to directly control it myself + it makes the healer more responsive to healing.
So, I can do TDS T3 trash with just my shaman healing, but TDS T2 and up, I run my shaman's DPS INI/spell-set + a healer merc and am pretty rock solid tanking. With the DPS INI, my shaman is going through debuffiing, DoTTing like a mad man, making sure every buff is up, including all the short duration ones and support heals great.
For my tank, any one healer is not enough after a certain point, so I never really thought of my shaman as a primary healer and it was more about how well he stacks with a merc healer.
I think any priest class will easily do support healing, but if you want to go as far as you can with only one healer, well that choice is easy. Go cleric.
my group was shaman sk mage cleric wiz and enchanter and I had no problems whatso ever doing tds ha or tds progression or anything before it expac wise, I know some will say they overlap hopwever I had the shaman doing the slows and debuffs and the chanter doing the mezzing and dps and it was total win! druids are cute loved them one of my favorite classes but just cant see druid in a tank group when you can have shammy buffs for a tank .