So I've been doing some testing with the AON armor scheme, going full AON. EG only armoring the Deck, Belt, Turrets, and Secondaries, 0 armor on the DE or BE locations. Full immunity to my own guns 10K and out.
1) You obviously take a ton of chip damage from AON. 8" and under tend to arm in the unpotected areas, and capital guns overpen (as you would expect). What's interesting is I've noted a number of Mid-Deck penetrations at ranges of say 14K. Inspecting the damage model, it seems to be calling the superstructure pens (oustide of the armored conning tower itself) as mid deck penetrations. Since my ship is immune to plunging fire at such ranges to the deck (I checked the oppositions guns) This leads me to believe that it is actually the extended deck providing this protection. Since the superstructure is so large, this is a HUGE weakness.
2) Second is the critical problem. AON doesn't actually protect your ammunition stores. I have been successfully detonated from an ammo explosion, via a penetrating hit, to the stern belt extended. I would construe this as meaning a bow, deck extended, ammo penetration is possible as well. The point of AON is to focus the protection into your critical areas, EG the machine spaces and ammunition store/barbettes. Since it's currently not fully protecting these areas, then any ship designed around AON is a tinder box waiting to happen. See photo below.