I personally think that 326 PPI would've been fine on the 5.5" iPhone 6 plus, but that Apple caved to the pressure of getting to 1080P and >400 PPI screens.
Have you actually compared the 6+ display to the 6 in person? The difference pixel density is fairly easy to see.
It was only a few years ago that Apple were the leader in pixel density. It's good to see them catch up. In a few years all of their devices will be >400 ppi - I can't wait.
btw, benefits stop somewhere around 450 - 550 ppi depending on a person's visual acuity. We still have another generation of improvement until Apple devices reach the limit.