Kat is not OP, but her effort: reward ratio by far too low
She is not overpowered, she has counterplay, but it's really unfair that she gives so much reward for so little practice, when 90% of midlane champions require skill to be effective. Easy mode champions should not be this good. Imagine if Garen is as good as Renekton, or Xin Zhao is as good as J4, its not fair.
She needs nerfs, maybe not to her damage, but if she is to keep her current damage she needs to be more difficult. The change in the latest patch is a step to the right direction but its not enough. I think a reasonable change would be to raise her passive reset time from 15 to 20 seconds, while increase her ultimate CD from 90/60/45 to 120/80/60 seconds. This way Katarina requires the same amount of resets to have her ultimate off CD, but if she doesn't get a reset, she is punished with a longer CD.