Pretty limited use for a feat, but the effect is unprecedented - at most, max DC adjustment from a feat that I’m aware is +2. Most are +1. Half Cha mod to DC can be pretty high with magic.
I’d almost rather see the scope expand to include all enchantment spells (rather than just charm subschool), the bonus drop to a flat +1 or +2 and add some other effect like increasing HD effected by Cha mod or something (e.g., sleep only effects 4 HD currently, but this would make it 4+Cha mod HD. Or maybe reduce the penalty to charm DCs when target is threatened. Or give bonuses to checks to convince charmed creatures to do things they wouldn’t normally do.