My thoughts on this are similar, but with a slightly different direction.
In my group, we have added 2 new skills, thaumaturgy & faith. They are used for the casting of arcane & divine spells. They set the DC to resist the spell, which makes a high-level caster harder to resist.
On a natural "1", the spell fails, but still takes up a spell slot. On a natural "20" the target gets no save (spell resistance still applies).
This seems to work ok, so far.
