Does AMPDU bring some extra benefit or is it just another method of marking packets?
A-MPDU is a major factor in wifi performance with large volumes, and will reduce the 802.11 overhead dramatically.
In MT it is only enabled for priority 0, which is all traffic if priority is not set by FW rules. It should be on for all priorities, unless there is a reason to leave it off.
Leaving it off for priority 6 and 7 can be justified because voice is normally low throughput, but very sensitive to response-time (no buffering allowed).
But if you want priority and throughput, I would put it on.
http://forum.mikrotik.com/t/help-mikrotik-capsman-wireless-download-speed-max-200mb-but-pcs-mobiles-link-speed-is-866mbps/142677/1
The youtube session may be a bit long, but most information and calculation spreadsheet are included in the posts that follow, like …
http://forum.mikrotik.com/t/help-mikrotik-capsman-wireless-download-speed-max-200mb-but-pcs-mobiles-link-speed-is-866mbps/142677/1