So basicly Fiendish codex gives 35 Ap 
Athene's unholy grail after being completed gives 30 AP (Not counting the passive) 
Even if the passive compensate , You shouldn't lose base stats when completing an item
I saw that on twitter and since no posts has been made about it I decided to do it myself
Yes. Let's completely discount the passive which at minimum gives 20 AP.
These posts keep coming up and they are all stupid because you are too lazy to do math. The item could literally read "50 AP" and then Dissonance would be "5 AP for every 25% mana regen from OTHER SOURCES" and the item would be the EXACT same. Net zero change.
This was a thing back when 
used to build out of old 
. Upon completing the item you lose the "upon levelling up restore x% hp"
The 
items do the same thing, upon completing the builds into 
or 
you lose the headhunter effect.
Same applies to 
items, when upgraded you lose the "heal after taking damage" passive.
They're not stats but I would say they fit in with the 'loss of bonuses' topic.
As you said, this only works if you discount the passive. With the passive counting only stats that you get from Athene's, It adds up to 50 Ability Power. More than the sum of its parts
I think should give 40 AP without the passive.
btw riot recently patched it out but you could just rush this item with mid lane smite and then build 5 
and get to like 500 ap at 10 minutes.
So basicly Fiendish codex gives 35 Ap
I saw that on twitter and since no posts has been made about it I decided to do it myself
looooooooooooooooooooooooooooooooooooooooool
