As somebody who mained Viktor for a very long time and paid particular attention to many of his matchups, including Katarina, I have this to say.
On the original topic, is Viktor OP? i'm not sure I would say that he is sleeper OP, I think he is strong and much stronger than his low rate of play would seem to indicate but I agree that there are other champions that do a lot of what he does and do some of those things better. Viktor's teamfighting is very strong, where he can stun several people and deal tons of damage to multiple targets if played right -- this is one of the most important reasons to play Viktor. His pushing in the midgame is better than everyone else's in the game, bar absolutely nobody. With augment+deathcap He can clear a wave of minions with one cast of his E. Of course, he has many serious weaknesses as well, including his lack of mobility and reliance on successfully poking with death ray.
I have spent a lot of time cataloging some of Viktor's lane matchups. Katarina is a really interesting one, there are a lot of intricate mechanics that go both ways in that matchup, and I think if both parties play absolutely perfectly and the lane existed in a vacuum (e.g. ganks don't sway the balance), Katarina will win the lane, but the matchup is extremely skill-driven. The reason Katarina should win is that Viktor's cooldowns, especially their ults, are much more prohibitive than Katarina's cooldowns. Viktor has a lot of disruptibility against Katarina, but the more reliable disruption for Katarina's ult is to turn around and cast his ult, which instantly stops death lotus. If he has to use gravity field to stop death lotus, he'll likely still take most if not all of the damage from that skill. Katarina has to bait the ult and then get into position to ult again before death lotus is on cooldown. Viktor can make this hard by outpushing her and playing safe when both ult and flash are down. But ultimately, his cooldowns leave him in a very vulnerable position whenever this is true.
