I did achieve 866 mbps with SXT 5 ac
802.11ac 3x3, 80MHz channel, SGI, VHT9 (
http://mcsindex.com/) can give you more than that for connection rate. I am sure you are aware that connection speed does NOT equal throughput. Most users don't care what the connection speed is, but rather how much data they can get through the link. And even if you get to an all 3x3 or 4x4 configuration, other factors could prevent you from achieving these connection speeds.
I have two laptops - both with 2x2 802.11ac radios, to a 3x3 802.11ac AP. One gets 480Mbps of tcp throughput (Mac with broadcom), the other about 200Mbps (Dell with intel/ Win7). Both indicate connection speed of 867Mbps. I propose that connection speed is only one of many factors that determine actual data rates.