Having saveorX effects deal stat damage seems the best to me. Going 4E makes them too insignificant and worthless, while time spent modifying every such spell'd be enough to design a whole new magic system.

But flat xdy damages that'd work for those with massive scores would be instantly fatal with those that are not. So division is the way to go. Get spells to halve the relevant stat when cast, then again each round until 0. The average guy then has 4 rounds until death/suck. Something with a huge score like 36 will have 6 rounds.

To prevent undue suffering, don't change stat mods until spell's finished/dispelled.