Is an item that increases spell save DC balanced?


One of my players’ characters likes to use illusion spells in combat. To improve their effectiveness, he is asking me for an item that increases his spell save DC. The Rod of the Pact Keeper provides a spell save DC boost. However, it is locked to Warlocks. I’m considering giving him a similar item without the class restriction.

Example:

Ring of channeling (very rare)
While you wear this ring, the spell save DC of your spells is modified by +2.

Would it be unbalanced to have an item provide just a bump to Spell save DC?

If it is relevant, he is a level 15 Sorcerer and already has Charisma 20.