No, those passives increase weapon damage, not spell damage as the passive tool tip states quite clearly. Slotting it is a bad idea for another reason too. While ultimate abilities base damage does scale on your highest stat from maximum stamina/magicka and weapon/spell damage, their damage does not benefit from CP if they are put in the wrong places. For example on a magicka build those would be in Elfborn, Elemental Expert, Spell Erosion, while dawnbreaker does physical damage which gets boosted by Mighty and Precise Strikes. Taumaturge boosts both, since it affects all DoTs and dawnbreaker has a DoT component. Probably your class has a cheap ultimate that does magical damage you can use instead.