At higher levels, a common situation is to have "binary" saves. Classes with the good save progression are generally also ones that get good advantage from having a high stat that governs that save (e.g., Wis for clerics and Will saves). As a result, you end up with situations in which, if the cleric has any chance at all of failing a Will save, the fighter, rogue, etc. all automatically fail -- even if they took Iron Will and didn't dump Wisdom. They just can't keep up, because the discrepancy, even not considering attribute bonuses, is nearly a third of the range of the random number generator itself.
I wonder at the effect of giving everyone a flat progression in all saves: +1/2 levels. The difference in attribute scores will still provide a meaningful difference in the range of bonuses. I'd then recommend merging Iron Will with Improved Iron Will (for example), so that the single feat provides both a numerical bonus and a reroll chance.
The end result is that the cleric and monk will still have the best Will saves, but anyone else who invests the resources will be able to at least come close.