Personally I don't think she's overpowered. She's strong in teamfights if she isn't locked down, apart from it, she's not really that strong. Despite that, sometimes it's extremely hard to stop her ultimate due to its high damage (before you're even able to apply your CC).
Currently Katarina's ultimate does 35 / 55 / 75 (+ 25% AP) (+ 37.5% bonus AD) magic damage per dagger. She throws a dagger each 0.25 seconds, for 2.5 seconds (total of 10 daggers).
Her ultimate could receive a small nerf just by changing how the damage is applied. Rather than having each dagger do the same ammount of damage, reward Katarina players for getting good ultimates (forcing the enemy to be exposed for the longest possible to really get a lot of damage).
Katarina would still throw 10 daggers, one every .25 seconds. The daggers would differ in damage:
1-5 Daggers : Each Dagger does less damage, maybe, a 50% reduction (Half damage on the ulti for the first 5 daggers)
6-7 Daggers : Each dagger does full damage (Like currently)
8-10 Daggers: Each dagger does 150% damage (More damage, but it means you're taking damage for the past 1.75 seconds (plenty of time to stop her ulti or flash out)