[2019] impale and armor interaction

Could someone please explain how impale interacts with armor?

Consider a char with 100% chance to impale, 0% increased impale effect, and 5 max impales:

- 10% of your phys from tooltip is stored to a monster when you impale it

- each subsequent hit, up to a max of 5 hits, deals the aforementioned 10% as reflected phys damage

Is mitigation from armor being applied when the damage is stored or when it is dealt? And because of how increasing values of armor penalize smaller values of damage dealt, is Impale really as good as its made out to be?