Data speed in wifi comes in 2 different definitions:
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Interface rate. The rate at which data is leaving the interface. This is theoretically defined, and Mikrotik is no exception. List of attainable interface rates can be found here: http://mcsindex.com/ .
You just have to understand the Spatial streams (2S = two streams), MCS encoding (for ac from 0 to 9 per stream, for n 0-7 for 1 stream, 8-15 for 2 stream), and the 0.4µs (SGI) or 0.8µs guard interval. The used MCS and stream and guard interval depends on SNR and on CCQ. (Low SNR or low CCQ will make the interface step down in streams and encoding).
So for everyone (Mikrotik and other brands), with no exception, 80Mhz/2S/SGI will give 866.7Mbps interface rate with a good signal. -
Payload data throughput mainly depends on the interface rate and the overhead. That overhead depends to a very large extend to the buffer size used for the transfers. Buffering happens in A-MSDU and in A-MPDU. For 802.11n and ac the maximum buffer size is known and implemented by most. (Mikrotiks limits to medium sizes only: the A-MSDU to 3839 bytes, and the A-MPDU to 262143bytes) http://forum.mikrotik.com/t/ac2-vs-ac3-wifi-not-over-200mb/148289/1 . The overhead can be calculated for any buffer size. CAPsMAN typically reduces the A-MSDU further to 2048 by default.
There is a 3th moderation of speed and that is the TCP congestion avoidance control. This is not related to the AP, but can be influenced by it, So on this 866.7Mbps interface rate, TCP is typical max at 290 Mbps, while UDP is up to 390 Mbps for Mikrotik. (medium buffers and other driver inefficiencies).
Of course any other AP or client on the same channels will consume some part of the air-time, and all other AP’s and clients will wait. If channels are different but overlap they will not wait but destroy each others transmissions and the CCQ will drop, and so will the interface rate used.