When designing anything that affects a save DC, the target that should be considered when determining rule balance is the target most likely to be affected by the spell. That means that in the case of a spell with a Fort save, the conspicuous target is a character with a "bad" Fort Save.
"Bad" saves lag 2 to 3 points behind "good" saves, or roughly six levels. So a +1 to a save DC is the same as six levels of saving throw bonuses against the conspicuous target. +2 to a save DC (GSF) is worth 9 levels of saving throw bonuses.
What that really means in practice (because most encounters match the PCs against opponents of roughly the same power level) is that casters with SF and GSF have a 5% to 10% advantage when casting spells at conspicuous targets.
That's a huge, practical advantage in game for the use of SF and GSF. With the old versions of the abilities, not taking SF and GSF was almost always a mistake (and there's nothing worse than a feat that you absolutely, positively should always take - that's what class features are for.) Even with the revised values, pure spellcasters will probably still take them, because the advantages they offer are so significant.
