I'm noticing some issues with spell damage foci and worn effects stacking. I'm testing on my Mage (Feydakin) with Comet Strike Rk. I (Fire-based).
True Fire (80% to Fire-based) doesn't stack with Brilliance of Fear (15% worn v1 spell dmg) or Brilliance of the Elder (10% worn v1 spell dmg - for redundancy sake). If I unequip my foci and leave a Brilliance item on I receive the listed % increase. I always assumed they stacked since True Fire puts the bonus in slot 1 and the Brilliances are slot 12... or have I been wrong all along and this is working as intended?
Faerune II (5% worn v2 spell dmg) doesn't work for spell dmg at all. With all worn dmg/foci items unequipped spell damage is exactly the same with/without Faerune II item. Edit: It seems like it also doesn't affect melee dmg as a quick test with my Rogue (Daggar) shows 1566 max hit (white hit spam) with his mainhand with/without his Faerune II equipped.