Ok I admit the healing spell would be op but fireball would be fine as long as it's damage is nerfed from 1 to .5 and seriously how could it cause any lag....
The whole reason for someone to use the fireball over the bow is that if it ran off of the mana system then you have a regenerating ammo pool though spamming wouldn't be too good of an idea because you would run out of mana and have to recharge, which isn't exactly easy to do in a pinch. It's simply a choice between a hanful of fireballs that regenerate vs 200 rapid fire arrows that need to be replenished by "artificial" means such as pickups and vendors.