ive said for a long time (albeit probably not on these forums) that mage damage in general should have been slashed MASSIVELY and +spell damage should have been added to a wide variety of equipment.
as it stands right now, 30cl gives you the "base" mage damage which is by itself, quite strong. when you stack the +% spell damage items, it gets bapted up really fast.
you can get around +70-80% spell damage with the right equipment, though many are limit 1-2.
if you manage to get access to a dragon orb aswell and throw on mind afire, which stacks with dark accord/potency... the result is just pants on head retarded.
i personally never got to the optimal item set, as some of the +spell damage items i've factored in my math weren't in game then - but even still, it was possible with legendary dark accord, sinister barrage and rend to output in the vicinity of 600hp rounds with ~+50% spell damage and mind afire. with the optimal equipment, it would probably be more like 700-750, perhaps more. i also was not a gnome (old races).
noone really listened to me then, and noone has had all the items to get to that point since i played isabella to my knowledge, nor have i personally had the desire to. in my eyes it would have been an abuse of game mechanics and generally lame. regardless whether or not it is used it should not be possible for any character to be able to output that amount of damage in a single round on one target
it shouldn't have been left possible, though if i was some evil dickwad and went around 1rounding 700hp tanks i'm sure it would have gotten more attention.
the way that +spell damage scaling works is messed up, and needs to be looked at in general.
if you need a list of the items that give that stat, hit me up in a message - but im pretty sure you immortal types can just search that stuff
edit: thats lame that the forums edit the f bomb into bapted. i like the fbomb :(