So +0 cha gives you a 15, or +2 modifier. Which means 2.5 feats to 20 cha, or level 12 before maxing the stat.So a 1 point difference in Cha caused a character to fall behind the power curve of the group...?
I think that's your problem right there.
Fix the 1 point if you want, but I would be more worried about the concern of the power curve myself.
While you may think maxing the stat isn't important, by level 12 some foes can have +10 in a save.
A 20 cha gives a save of 17, vs 16 with 19. So a "hit" rate of 30% vs 25%. That is big. With metamatic, that becomes 49% vs 44%; so the 20 cha sorcerer is 1.1x to 1.2x better at landing the spell.
It hurts. I mean you aren't nerfed to "barely better than a commoner" but...