For the most part, isn't mana gear useless? (Let's exclude the conjurer, who plays dress up right before conjuring.) I can think of very few occasions where I ran out of mana when I wasn't ranking. PK fights are way too short.

So, mana gear is worthless in PK.

If you want to consider some kind of gear based spell enhanced damaged, how about modifying based upon total mana and/or total mana capacity. Normal spell damage might be the mage's naked mana total and the modifier be based upon gear based mana.

Woe is he who faces the the 2000 mana pillar of lightning. Of course, that poor little drow mage might have to drop his HPs down, so the perk would have a cost.