Can’t confirm this without doing some more testing, but after swapping 5 points of “Reverence of Duality” (+10% HP, Mana, Damage, and Healing) to 5 points of “Heavenfire” (+15 Spell Damage with a Shield), I do not appear to be dealing any notably different amount of damage with the Smite skill.
Even accounting for a possible double-dip of +Fire and +Spell damage on Smite (resulting in a +20% adjustment overall), I would imagine that the +15 damage should be added to the base Smite damage before modifiers, meaning that the 450% added fire damage and 70% added spell damage I would normally have (without the bonuses from “Reverence of Duality”) should result in more damage overall.
To illustrate:
Smite has a base Fire damage of 30. Let’s leave that unmodified by anything but the above values for now. It has the Fire, Spell, and Attunement tags, but we’ll ignore stat-scaling for now.
- With Rank-5 “Reverence of Duality”: 30 x (4.6 Fire + 0.8 Spell) = 162
- As above, but single-dip +10% damage: 30 x (4.5 Fire + 0.7 Spell + 0.1 Damage) = 159
- With Rank-5 “Heavenfire”: (30 + 15) x (4.5 Fire + 0.7 Spell) = 234
As you can see, there should be about 44 to 47% more damage dealt with the adjusted base spell damage, provided I’m understanding Added Spell Damage at all correctly.