[Gef]

I'd say Kelly answered this question thoroughly. From a world-building standpoint, if you don't like the idea of mana as a pervasive resource and requirement, you might substitute a gadget. All wizards must have their wand or holy symbol or medicine bag; these work out to be about -40% as Gadgets. Or they have to drink periodically from the Fount of Power, or dragon's blood, etc. Depending on how often, this is probably easiest to model as a Pact limitation.