Why would the save bonus have to outpace the spellcasting bonus instead of simply keeping up with it? I'm not clear about what you're trying to achieve.
High level spell/effects are scarier than low level spell/effects. If Save-or-Suck is limited to "eat a mild penalty/lose a round or two", the effect can land over 50% of the time. If Save-or-Suck/Die climbs up to "removed from combat/dead", then a level-appropriate target needs to solidly expect to save. That means that, at low levels, level-appropriate targets should save a bit under 50% of the time. At high levels, level-appropriate targets need to save almost all the time. In other words, saves need to get more reliable with level. This may not be "fun" for the spellcasters, but it is the penalty for even *having* Save-or-be-removed-from-combat scale effects. The "not-being-fun" aspect is part of why 3e changed the saving throw system. The need for easy-to-make high level saves became obvious shortly after people starting playing 3e.