I was thinking exactly along those lines. It seems a great bending of the rules in terms of raw damage output to create a spell like a mini-gun - and hose the opponent with it. Good to know its considered bad form.
It does make me think that a boosted version of the V's Sling rewritten at level 20 for +15 damage is nasty, and especially if the Magus then also starts multicasting the spell. An opponent might survive one casting, but being hit three times in one round by +15 damage is likely to hurt a lot.