I wonder how much of the cleric's apparent power is due to his ability to cast self-buffs such as divine favor, divine power and righteous might. If almost all the cleric's spellpower had to be used to heal and buff other PCs (which is supposed to be his primary function anyway), would you still consider him to be overpowered?
What if the cleric could only cast a spell that only affects a single target on himself (healing, buffing or any other spell) once per day for every odd level that he has attained (once per day at 1st level, twice per day at 3rd level, three times per day at 5th level, and so on), and an additional single-target healing spell on himself once per day for every even level (once per day at 2nd level, twice per day at 4th level, three times per day at 6th level, and so on). This will help reinforce the point that a cleric's role is to heal the other party members and to make the other party members perform better, or alternatively to heal everyone and to make everyone perform better with spells such as bless, prayer, mass cure wounds and mass bull's strength. Would the cleric still be overpowered then?