You do realize you get around 200-250k healing for only 6k mana right? It doesn't look like a lot because it's small to each person, but if you do a single burst and check your meter you will be surprised.
Shrooms are just as good as Wild Growth. The trade off is wild growth is smart and has range but costs 2x more, shrooms are short range, not smart, have a worse setup and are cheaper. Same healing done per person (with my gear it's about 25k unbuffed).