Hendrik,
Yes, please work with TAC on those ARM.
I was triggerd by the following: "the APs are all installed down a passage way all in a row (line of site) - we have the odd user that would report if they are situated close to a window (basically the furthest area from an AP) they struggle to connect and they get disconnected.";
In general, it is probably the worst choice to mount AP's like this, in a corridor, all in line; specially if you need coverage in the adjacent rooms. Having line-of-sight will cause a lot of co-channel interference, and result in poor performance, coverage and user-experience. Place APs in the adjacent rooms, so they are 1) close to the users and 2) non-line of sight as much as possible.
You will see that ARM in these conditions will turn down the power, resulting in poor coverage in the edges; overriding ARM and setting the power to the max will give the user it '5-bars', but poor performance. It may even be that if you turn off all AP's and leave 3 or 4 on (at least on 2.4GHz); on 5GHz you have some more channels available, you will have better results than with all the AP's.
Be prepared to work with an RF planning/troubleshooting specialist if you can't fix this with ARM; or results are still poor after the changes.
This document may help you as well: http://community.arubanetworks.com/t5/Validated-Reference-Design/Indoor-802-11n-Site-Survey-and-Planning/ta-p/155136 ; it is for 802.11n, applies very similar to 802.11ac.