Her damage is already the highest off all not-mage supports statistically, and thats with outsourcing some damage on allies(if your adc uses the Q damage proc, it counts as him dealing the damage for stattistics)http://champion.gg/statistics/#?sortBy=general.totalDamageDealtToChampions&order=descend&roleSort=Support
Her damage is perfectly fine, her overall performance seems also to be quite well, her winrate increases towards 40 min, so her impact outside off lane seems to be quite high aswell.
She's a beast in aram, which suggests that her scaleing with gold/xp is pretty high and her teamfighting/siegeing is awsome. I guess most sonas invest to little gold into AP, especially sheen(yes, i said it, supports should sometimes invest gold into ap!(obviously after gp/5+sightstone))
The only real problems of Sona are:
- no big cc pre 6, so little synergie with ganks till then
- very vulnerable, squishy, low mobillity, no hard cc pre 6, rather big hitbox. Combined with the high popularity of skillshot->all in supports like nauti, blitz and leona, this is a huge problem for her
Her winrate vs blitz is only 46.6%(compared to 52.3 overall), 48% vs nauti, but 58% vs annie, 59% vs tham, 57% vs shen/tarik/bard
I honestly don't think she needs buffs, maybe add some more self-defense to her, but slightly lower her damage to make her a bit less risky, but any straight buffs would end with her beeing broken if she plays in a good matchup, and probably beeing overall to strong.
Also, whenever i play her my self/play with her, i don't feel like she lacks damage in any form or is weak, she just is pretty damn dead if she ever floats out off position...