Based off the way you explicitly deny double dipping of mitigation/damage taken modifiers, it sounds as if the bonus damage from hitting an Impaled enemy could be double dipped by damage dealt modifiers; like "physical damage with weapons", support gems and so on.
So following that logic, an Impaled enemy will grant added flat physical damage to subsequent hits, similarly to Despair causing cursed enemies to take additional flat chaos damage.
If it is flat damage, this would also mean that you could infinitely ramp up damage on Impales, assuming you always hit the enemy before prior impales ended. Because the Impale bonus damage would contribute to a stronger Impale on subsequent hits
Is this correct? Or have I missed something?