There was a fair bit posted after PoP and even more so after GoD came out that indicated a lifting of the "soft cap" on ac.
As I understand it it wasn't actually a cap but the ATK rating of the mobs meant that over around 1250 the return diminished so rapidly that it appeared like a cap. I suppose that is one definition of a "soft cap" - a quirk of the maths.
With mobs having higher ATK in the new expansions the higher ac started to be useful again and effectively lifted the cap.
Now there was something from the Summit about them firing a programmer who put silly ATK figures on GoD mobs and adjusting them down. This may have altered things yet again.
I post in the sure and comfortable knowledge that Gbaji will correct me if I'm wrong
And neverdone there is a massive difference between 65 with 400AA and ele+ gear on a server with virtually no No Drop. and what a 56 can achieve with the best droppable stuff on normal servers. I have healed the guy - I don't have to ask him his stats.