I've been tweaking the numbers in each build so I don't remember the exact numbers and it also varies for each type of buff (purple leaves can be for melee, ranged or knockback buffs, we'll likely do more icons to help differentiate)
for melee I believe its +0.4x dmg per activation, so if the purple leaf plays twice, for example, your melee attacks will be doing 180% their normal damage
overall I'd say theyre very core for a good build