Perhaps a diminished armor or physical resistance would be more thematically appropriate. Or -1 to bodybuilding and willpower. 50% less of a bonus is significant, but it is still powerful. But at least you couldn't cast two good spells AND a weaker spell or move in a round. A lot of the time you'd want to use your extra AP for defensive positioning or spells like invisibility, especially if GC also increased the chance enemies attacked you.
