Synastry is a bit op.

[chumsy]

That's... not how math works. Functionally, Divine Seal increases your Healing per Second by 30% regardless of how many targets you have. Synastry technically increases your Healing per Second by 50%... if you're not healing the Synastry target and you're not using AoE, which greatly reduces how powerful that 50% actually is. If you're trying to burst heal the tank for something in the rein of T13's Akh Morn, for example, Synastry does effectively nothing for you.
Synastry is really good, but it doesn't outclass the other healing output buffs at all.

functionally, Synastry only heals 1 extra target, and functionally divine seal works for all healing spells, aka AOE heals. So functionally Divine seal is way more powerful. You dont really need math to know that. Yes I understand that if the other target takes no dmg his effective healing is 0.