I'm staring at my iPhone 7 at the same PPI as this and I'm frankly baffled. I don't have superhuman eyes, just normal decent 27 year old eyes, and the jaggies around text and round objects are very clear to me at a comfortable distance. Look at a notification dot. I'd be shocked if anyone with normal vision couldn't tell that's not a perfect circle.
I've had people flat tell me I was lying about this on the internet, but it's in front of me, I can see it right now...
Anyways, had beyond 326PPI not mattered, why do the Plus at 400 and the X both look so much sharper? Particularly the X does, since it doesn't have to downscale from 3x to 2x like the Plus.
Heres my question for you:
So what?
Can you still tell its a notification? Can you still read the text? Then you're good.
Obsessing over a fraction sharper text, which doesn't actually get you anything real advantage wise, is pointless.