At least some of you have wondered whether there’s any difference between a property usually referred to as Spell Damage or Spell Power, as seen on, for example Spirit of Aquementas:
Equip: Increases damage and healing done by magical spells and effects by up to 20
vs one of the Raw Element Damage coming from “… of Wrath” items, such as this one Duskwoven Branch of Arcane Wrath:
+20 Arcane Spell Damage
The TL/DR version: They are exactly the same – either one is multiplied by the spell coefficient. EDIT: Footnote: this post aims to clarify the “of Wrath” property, not the standard ‘up-to’ spellpower
The spell coeffecient is very easy to calculate. A spell of 3.5 seconds cast gains 100% spellpower. So the coeffecient is calculate using (untalented cast time / 3.5)
For spells with a casting time the idea is no matter the speed of the spell all benefit the same: you can cast faster spells more often so you receive a proportionally smaller damage bonus. It’s similar to how attack power makes slower weapons hit harder so the DPS increase is the same.
It just starts getting more complicated with HoT/DoT spells, and talents. For example a 0.5-second reduction is more powerful for Wrath (2 second cast) than Starfire (3.5) because for the same damage bonus one is boosted by 33% (2/1.5) while the other by ~17% (3.5/3).