Katarina actually IS S Tier on Masters.
She holds an AVERAGE 58-60% Win rate among high elo players, and consistently performs in all elos. And has for a couple years now. However, the reason this doesn't get much notoriety, is because people are honest about playing her: If someone cannot do the 1v5, they don't play her. If they can, they play her almost exclusively.
At lower elos her statistics get hidden by people trying her out, who do poorly and have trouble grasping her dagger cutting and casting multiple skills, shunpoing allies, and figuring out how to dive correctly.
In other words, they PAUSE.
High elo Katarina doesn't pause. You can't hit her with any sort of CC outside of a point and click, or an on hit CC. Skill shots will almost ALWAYS miss, because unlike other champions who are vulnerable to CC, Katarina can kill you before it hits, if you let her skill first and then try to CC her prone (Master Yi on the other hand has to get a few hits in, so you can successfully land skill shots after taking 1-2 hits without risking his alpha strike)
High Elo Kat will also almost always exceed her Shunpo Range by flashing into it. This EASILY puts her into a position where no one can skill within range of her without being in kill range.
Kat is one of the 4 top win rate champs in the game among players who know what they are doing.
