"You may have an issue because all those APs are in the hallway and they can hear each other too much vs if you had them inside the offices.
Whats the structure of those walls ?"
^^ This!!! We have an extremely dense deployment of Aruba with 4 of our sites having over 250+ AP and 3000+ clients daily in each. Here are some suggestions I would make.
Don't let APs see each other directly if possible. This means putting them in rooms and not hallways. When they are in the hallways, you get several bad things: users are on the other side of the wall(s) so they get reduced signal, APs will turn power down (see first point), interference indexes calculated by ARM will be higher than normal thereby adding possible issues with ARM, etc.
Turn off at least 1/3 to 1/2 of the 2.4ghz radios (site survey needed for specifics).
Turning off HT-40 really helps in many ways (you've already done that).
Be careful with adjusting power levels. Try to keep power levels fairly close for max/min. We go as low as 9/6 in our 'ballroom' while some high density areas are 12/9. Our normal levels are 18/15 for the 4 really high density sites mentioned above. If you use a larger spread, say 18/12 then some APs will be 18 while many are 12... this causing a clumping of clients trying to get on the AP with power level 18 (it can happen but may not).
If you start lowering power levels then consider removing some lower data rates. This is an art more than a science so lots of testing will be required.
Get the Aruba document(s) regarding how ARM calculates interference indexes (intf_index). You can figure out a great deal with that information. You will be able to determine when an AP should change channels and what channel it would probably change to as well. Airwave can help with visuals as well.