Trinket is all overhealing is divided among the 3 closest people that need healing, pretty similar to the legendary cloak in MoP. So, when you make it stupidly big, of the 10m regrowth, all of that overhealing will be divided among 3 other people that need heals. So it should instantly top 4 people at once. Mix in PotA, like you mentioned, and that will be 2 more targets with overhealing, then split to 3 more people EACH. So, you'll have a 21x effect Regrowth (~10mil for me) split among 4 people and 2 7x Regrowth (~3.3mil) split among 4-8 people.