Honestly if I were SE I would limit healer DD buffs to be strictly single target on the DPS themselves. So long as some classes have AoE DPS buffs and others don't it'll be impossible to balance healer damage between both 4 and 8 man content because of the effect being doubled in 8 man. Either Whm will have enough personal DPS to compete in 8 man which in that case Whm will obliterate everything in 4 man, or Whm will be balanced for 4 man, but will under perform in 8 man since their personal DPS won't be enough to catch up to the AoE buffs.

The better to balance around in the future way to "nerf balance" would be to instead remove the ability to AoE the cards in the first place where the cards are only single target (and stratagem would buff 1 ally instead of debuff the monster) and then personal DPS and DPS buffing should be able to be balanced.