[Schortt]

To quote the info in the wiki:

"As of the 5/29/2011 patch, monks will face an AC penalty for exceeding a weight of 14.9 stones, reflecting the classic mechanic.
This limit was independent of level.
As of the 8/13/2011 patch, monks gained an additional weight allowance, granted at levels 55 and 60. The exact amount is 18.9 at 55, and 22.9 at 60."

To echo what a few other people have said, AC does matter, but no one has done enough parsing in the right conditions (and published their research) to figure out where the hard/soft caps are at any particular level. There is definitely a difference in mitigation with sub 1000 AC and over 1100 (I have enough parses of a 60 IKS war vs a 60 Ogre war at various raids, especially Sky, to demonstrate this). I don't know if the difference would be greater with 1200 or 1300 AC or whether it makes sense to stop before then. One of my goals during the eventual Velious beta is to test some of this stuff more rigorously. Since boxing on the beta server will likely be allowed again (it was during Kunark beta) it will be a lot easier.