Willpower's universal +0.25%/point 'Overpower Damage' bonus, and gear's own '+Overpower Damage%' affix, only increase the SIZE of Overpower's own per-stack Additive bonus. A separate class of uniques/Aspects instead grant a true x% Global Multiplier conditional on 'while you have an Overpower stack' - a fundamentally different, more valuable bucket.
It is easy to assume all 'Overpower' text on a character sheet behaves the same. It does not: one family scales the stacking buff itself (still Additive-bucket, still subject to diminishing returns against your other additive stats), while the other family is an unconditional-once-triggered x% Global Multiplier that multiplies your whole damage output independently of how big your Additive bucket already is.
Follows from the affix_buckets distinction: 'Overpower Damage' affixes feed the Additive Damage Multiplier bucket (specifically Overpower's own per-stack contribution), while unique effects worded as an 'x%' bonus while Overpowering are their own Global Multiplier bucket like any other conditional x% source.
Overpower's baseline is 15%[+] Additive Damage per stack (disputed cap, see brk_overpower_stack_cap); Willpower and gear 'Overpower Damage%' affixes add on top of that same 15% base, staying inside the Additive bucket the whole time.
When comparing two 'Overpower' items, check whether the wording scales the stacking buff (Additive, diminishing) or grants a flat x% while-Overpowering multiplier (Global, non-diminishing) - the latter is almost always the stronger late-game pickup.
multiplier stackingLikelyAll
Open in the planner