866Mbit is the “air rate”, real data transfer you could get 400-550Mbit depending on conditions. here is another topic where people share the best settings for SXT ac:
http://forum.mikrotik.com/t/sxt-ac-best-settings/79554/1
Currently we are optimizing nv2 rate selection, please try it with regular 802.11 and see if this improves your results