Onto the main topic.
I am not sure if that feat is overpowered. It will be much powerful than most of the already existing WotC feats which adds more spells known to the sorcerer. Namely, Extra Spell.
But IMHO Extra Spell is very weak as a feat. And non of my players in my play group ever took it. It seems not worth spending a feat slot, especially after the introduction of Runestaff (MIC). Also, that feat does not allow a Sorcerer to learn spells which he cannot usually learn, like Arcane Disciple feat or Domain Access alternative class feature do.
Still, IMHO, Sorcerers need no more powering up. The class is already very powerful and versatile if the player chooses right spells and right feats. Also, using CHA bonus for determining Spells Known may cause some ruling issues. Because CHA bonus changes temporally by using spells & items, or by taking ability damages/drains.