Shaman are mostly interchangeable with a Druid so long as you have someone else to slow (Enchanter or Beastlord). With all that being said, Shaman perform much better than Druids as supplemental healers ...
First of all Slow is mostly mitigated, it doesnt do alot. Much more important that you aux alot.
Secondly alot of the incoming DPS is from stuff that you cant reduce with slow, DD, AE, Piercing...
As for being a better supplemental healer, are you kidding?
Druid can do hots to do a nice flow of healing on the tank, they can do a better big heal than shm's, and they can actually g heal and there g hot is usable, unlike the shm's.
Relic: Woundbane
1650 heal for 285 and 3.75 seconds.
hp per mana raito: 1650/285 = 5.78
hp per seconds ratio: 1650/3.75 = 440
Revitalize
1955 heal for 340 mana and 4 seconds.
hp per mana raito: 1955/340 = 5.75
hp per seconds ratio: 1955/4 = 488,75
So slightly less mana efficient (0.5%), but 10% better healing over time.
Don't get me wrong, i don't want the Shm to be a better g healer than the Druid, but please stop all the nonsense and old idioms that are no longer true, and get the shm back in the healing game, atleast just a little.