Shard of Gratitude will not synergize with this at all, because it's your MAXIMUM HP that gets reduced. Shard of Gratitude only heals your current HP, so its healing will not help you. Only Shard of Divinity and Stone Skin will give you back the sacrificed max HP.
I think I'll give its Momentum back.
Oh, and to prevent this from making mastery easier: now this decreases current HP as well as max HP. It works exactly like Shard of Divinity where it adds 20 to both max and current HP.