if you are using a 2x2:2 device in a 40Mhz channel with the 802.11ac PHY the max is 400Mbps datarate, you may disagree however it's WiFi science ;), you can check the datarates table on this http://mcsindex.com. You need at least 80Mhz channels in this setup to get 800Mbps datarates (not the same as throughput). However, there could be a lot of reasons why WiFi is not performing (interference, driver issues, congestion,...).
Florian I you are using a 2x2:2 device with 40Mhz channels and 802.11ac PHY you can have a max datarate of about 400Mbps (if the SNR is very good). datarates can be very volatile and are set on a per-frame basis. In reality is the 175Mbps real throughput not so strange because a lot of the airtime is used by the overhead of management and control traffic (datarates are not equal as throughput). Another sidenote, you don't share the airtime per AP, you share the airtime with every other device (clients or AP's) in 'listening-distance' (and this can be further then you think). You have to execute a spectrum analysis to check the channel utilization (wifi and non-wifi interference) to be sure there is no other device on the spectrum. Hope this could help.