Which is a bloody awful trade for an E6 game. You'd never get your second attack per level.
There are many instagib combos in E6. Fireball isn't a good one. Try Sudden Maximize, Sudden Empower Enervation. No save, just die for any of these classes. All for the cost of 2 feats. Or really pair that with any large damage spell. Dominate Person. Save or you're ****ed. The dragon gets unlimited 6d6 attacks in the form as breaths.As far as Fireball goes however, it's not a "good" spell because damage isn't a good use of a mage's time. In terms of damage it's absolutely dead on with multiple targets, 6D6 damage. Remember that's the same as a mage's hitdie, so you're potentially into instant kill damage against any E6 character who rolled poorly on their HP).