I am following the whole being a conductor and if you use it too far that it will cause damage.  I was thinking more of the lines of what happed to Arn at the beginning and keeps on happening when he uses magic too much.  which is he damaged himself due to lack of control and exertion.  I feel that if the magic is too powerful or if you have reached the limit of what your body can handle then you cause physical damage.

fire=burns
Ice=frost bite
telekinesis= concussion damage

so sort of like what you are saying. I feel like some sort of spiritual exhaustion should also be in order.  After all, you are utilizing your connection to Gia.  That could be the first warning, Second could be gradually increasing pain and finally physical damage.  This would follow the same for Ki users I think because though enhanced, their bodies are still just flesh and blood. 

Also, I am assuming mages could increase the amount of spells cast or the spell power over time? How would that work? Also, could they learn new spells?  I am assuming the Big Bad ass mage that destroyed the world was not always that powerful or the power that you are born with is that it?