The problem with +1 to DC's is that it is a +5% chance for the enemy to crit fail. Crit fail on a spell can end an encounter, while a crit success on an attack will just do more damage.
Having an item that only bolsters a single spell would be interesting, though, because it could be limited to less destructive spells. Like, crit failing Command isn't too bad, but Feeblemind...
"The target’s intellect is permanently reduced below that of an animal, and it treats its Charisma, Intelligence, and Wisdom modifiers as –5. It loses all class abilities that require mental faculties, including all spellcasting. If the target is a PC, they become an NPC under the GM’s control."
Imagine hitting the big bad with that.