Sona DPA Support

I feel that where other supports bring peel and defense she isn't the best but she can bring so much late game damage it is kind of scary. The build I have been using of has been followed by cause yeah no escape and vision OP. After that I build of some form, and then . I find that with her Q tacked on it is a scary amount of damage. at level 16 she can provide 500 (+100% ap) damage to AA for the team plus 30 additional and 15% increased attack speed. Ardent doesn't apply to her but for one AA from each team member (plus herself) can add up to 620 + whatever AP you have every 5 seconds. Add on two targets taking 200 magic damage +50% ap each and that's an additional 400 +100% ap. That means Sona can deal 1020 +200% ap magic damage every 5 seconds! Add in the crit chance of +50%, 20% bonus AP, and a passive for even more damage and you can see where I am going with this. Further late game can see her get and (I like the extra armor) throwing out because it's really not worth it to upgrade. I sometimes wonder with all that crazy base damage if building her further tanky instead of Iceborn and banner might be better though perhaps building the tank support item would be beneficial instead of spellthief? Is she a powerful hidden pick in the multiple tank meta where damage is lower except on 1 or 2 hyper carries? Is she the damage point assistant of lol? What do others think?