you are crying about a passive on a support? her damage isnt that over powered at all. look at the #s.
Sona base damage on a power chord Q is 40 magic damage +(50%AP) + 40% bonus damage
lets assume sona went 0-18-12 masteries and has thunderlord's keystone and her runes are flat magic pen for 8 magic pen, flat armor 9, flat mr 12 and ap quints 15 and her only item is
which gives her 5ap and 10 bonus damage on proc. on the proc of thunderlords she will do a bonus 10 damage plus 10% bonus ap
so she has 20 ap from runes and 
so for pwer chord Q would be 40+10(50% AP)=50+40%bonus would be 70
and lets say you as adc has base mr of 30 -8(sona's magic pen) would leave you 22 mr
and lets say she dont proc thunderlords or spell theif's passives
70 magic damage -22 mr would be 48 magic damage
now lets include procs from spell theif's and thunderlords
so we have 70magic damage +10 bouns damge would be 80 damage +10 damage +10% bonus ap (20*10%=2ap) so we have a total of 72 magic damage + 20 bonus damage - your base mr of 30 (-8magic pen) 22 so 50 magic damage +20 bonus damage
my math might be incorrect with the calculation of how bonus damage is received to target but 50 magic damage and +20 bonus damage dont see like its alot considering the has to proc on 3 passives (power chord on Q, spell thiefs proc , thunderlord's proc) and that with a basic ap rune page of flat magic pen and 15 ap isnt alot of damage.
now lets look at sona, she is squishy, has one of the lowest hps on the game, has no escape besides flash and builds no def stats.
typical ad in lane lvl 1 with dorans blade is about 620hp with 70ish ad with 5% life steal and base mr of 30+12(flat mr runes) of 42 mr how can she chunk you for 1/3 of your hp bar by herself?