Question about Ahri

When you're building correctly, Cassiopeia counters Ahri, right?

Here's my reasoning: -Poison is a pretty big area, and Ahri's speed boost isn't a dash. Using it, even if she walks away stops her from getting some farm. -Your main form of damage is targeted, and on a .5 second cooldown if she's poisoned. -She has to face you to use her skillshots, so if you don't waste your ultimate on the enemy jungler every single time they show up, you can easily get her with it. -If you actually finish your Seraph instead of building Aether right after finishing Tear, you'll have a shield and no mana problem. -Ultimate can prevent successful ganks. -Cass can reasonably build HP, since her damage isn't in a solitary burst of three abilities.

Reasonable downsides: -Immobile, so if you're the kind of player who gets caught by every Rocket punch from a Blitzcrank, you'll have a bad time. That said, if you're that bad at standing behind minions you probably should've switched lane with someone who knows how to play around a small handicap. -Conditional damage. Her damage isn't very good if you don't land the poison. Again, if you're the kind of person who misses a big aoe against a champion who can only dash if she uses her ultimate, you probably shouldn't be playing mid against Ahri.