Is stacking mana with the new ap itemization viable?

Of course it is, it has been viable before the buffs too, but mostly on lategame champs since you hardly have midgame pressure.

its always been viable even before this sonny boi.

Seraphs Embrace was a 120 AP item when completed before. Now it's 140 AP and ROA got a mana buff. It's ridiculous and honestly I would consider buying tear on pretty much any AP that uses mana at this point.

It's better on some champs more so than others.

It's core on Ryze. And I typically get it on Karthus.

but it has been strong before the buffs.