Originally Posted by
earthwormjim
Bonus healing as in, they retain what they do now, and they get extra.
The mushrooms receive a buff, this buff is what increases their healing. They still do what they do now, but they also get this buff from rejuvenation overhealing. You can go test the ptr right now, just bloom a mushroom without doing anything else, it'll heal you for 8-12k.
Mastery does not increase the healing done by them, however they do scale with spell power (I think that's just the baseline heal they currently do scaling). They also will crit (including the bonus portion), gaining the same crit bonus as every other healing spell.