The problem I see with a lock/priest combo is that you would be dealing with dots and those other spells they got, which I do not think fit well with the mage in there unless you drop the mage and do 2 locks and a SP then you have similar spells and will probably work good together. Problem still arises in that if you want to do healing with the SP then you have to go out of shadow and do them, not so with the shaman you dont have to worry about that at all, the boomkin would have the same issue you have to go out of boomkin in order to heal, now you can skip the boomkin and put your talents else where and still dps on druid and not have an issue trying to heal.