Sorry, I just read your thread. Little late I guess…
I think you still have some interference between your APs, but you also may have a severe hidden node problem:
https://en.wikipedia.org/wiki/Hidden_node_problem
http://forum.mikrotik.com/t/what-is-a-hidden-node/41915/1
Your APs can be reached well by the stations, thanks to the height and the metal roof. But the stations in the aisles between the racks cannot receive all other stations. If not already done, try “cts-to-self” or “rts-cts” hw-protection-mode.