I actually agree with that since if you are spammin abilities in the first place the extra dmg from atma's cdr and armor would come in handy the mana is not really necessary since you already have like 2.5k if you have Frozen heart but it gives more dmg from Manamune as well so i agree

Like your Build.
I wanted to write a build aswell. With nearly the same item build. But now its needless.

The only think i would change is infinity edge wth Frozen Heart. You get more dmg and also mana, cooldown reduction and armor. i think this would be better (:

greetings Zar (;