DC goes up at a rate of 1 every 2 levels. So a bonus to DC shouldn't go up faster than this. What is worse is that modifying a D20 roll is not the same as modifying damage.

If you want an arcane power attack, hmm...

In exchange for -1 DC, -2 caster level, and -2 to hit on any to-hit roll, you gain either +1d8 damage to one particular target hit by your ability, or +1d6 damage to each target hit (if a target is hit more than once, the +1d6 damage only applies to the first hit). You cannot reduce your caster level lower than the min level required for the spell, including any metamagic effects.

So a L 20 wizard could take a -5 DC -10 caster level to cast a fireball. With 30 int, that becomes:

A fireball would be:
DC: 18 15d6 damage to each target
or
DC: 18 10d6 to each target, +5d8 to one target

Or a L 20 wizard casting scorching ray at -8 to-hit -8 caster level:
+2 to-hit, 3 rays, each does 4d6 damage. The first time any given target is hit they take +4d6 damage.
or
+2 to hit, 3 rays, each does 4d6 damage. The first target hit takes an extra 4d8 damage.