I tested Magic missiles with +0% spell damage and with +36% spell damage. My max damage was 185-190 on Young Wolves.
Also, the tool tip says "Spell Damage". My assumption was that "Spell Damage" affects all spells, not a specific magic type like an element or arcane/magic.
My character is currently wearing 4 piece Tier 3 Scholar's set for +20% Spell Damage with no enchants. I also have 4 points into Devastating for +16% more Spell Damage.
I have tested attacking a Young Wolf with the following parameters:
No Magic Talents and naked: 200 Damage with Firebolt
With Talents and naked: 200 Damage with Firebolt
No Magic Talents and 4 piece Tier 3 Scholar Set: 200 Damage with Firebolt
Magic Talents and 4 Piece Tier 3 Scholar Set: 200 Damage with Firebolt
I would also recommend a change to wording then. The Character sheet shows "Magic Resistances" and then a subtype in that category is "Magic Resistance".
Power Caster verbiage can be interpreted as being All resistances that fall under Magic Resistances or read literally as the only magical sub-type "Magic Resistance". Below would be some recommended changes I feel would be adequate with the thought that this talent is to affect only a single sub-type of magical damage. Ideally, the other 2 talents in the intelligence tree affect all spells, it is a little odd that it only affects a single sub-type that has only one spell in it.
Spells like Magic Missiles are given a different damage type name outside of magic. A common default would be "Arcane". Then adjust the tooltips appropriately - that leaves the "Magic Resistance" table on the character sheet untouched.
list itemAdjust Power Caster's tooltip: "Spells dealing Magic damage ignore 40% of the target's resistance." Right now only Magic Missiles exist I believe that do Magic damage, so if that is intended for end-game. Be specific and state "Magic Missiles ignore 40% of the target's resistance".
Re-label the character sheet from categorizing all resistances as "Magic Resistances" and just label it "Resistances"