I have to disagree with you Emoto, Sona is overly powerful , yes her Q is her only damaging ability , but her kit is way to strong especially with the new masteries and the buffs to heals and shields.. but we wont get into that pile of worms , just take Sona as she is.
Hymn of Valor, (which is her only "dmg" ability) has a 50% AP ratio, and does a base of 200 dmg from and she only needs to dip into range to do it, which is pretty damn good range. as well hits two targets and will hit champs over creeps, so creep cover is not a option, its not a skill shot, thus if you only hit Q near a champion you can see, odds are good its going to hit that champion.
Her heal, has a 110+20% Ap ratio (+.5% for ever 1% hp target is missing) and gives a 115+20% AP +10/20/40 sheild on top of the heal.
But its not her abilities, or even the Masteries i suppose that make sona broken, its the cool downs, and power cord... with cool-downs of 8 /10 / 12 (without cd reduction)
with the 40% your looking something like 4.8 / 6 / 7.2 (not the greatest at math)
With CD, (not even 40) her abilities become WAY to spam-able, and her and her group get a host of buffs, and she does damage.. and unlike most "supports" she is set and ready in less then 10 seconds to do it again ..
Power-chord gives her a 40% boost in damage from her attack, not counting the jump from say Sheen. OR reduces your damage by 20(+2% per 100 ap) for 3 seconds, (and in a fight with someone who heals/shields, and who's only dmg ability is up in 4.8, 3 seconds is a long time not to have damage
you said she doesn't do as much bust as other assassins, Sona is slotted as SUPPORT, when is the last time you saw Alistar pull some of the crazy shit Sona can?