The 867Mbps are "interface rate" or PHY speed, the 150 Mbps are effective data rates. At high PHY rates the overhead is very large, 50% is no exception, and this is not by vendor but just the 802.11 wifi basic principle. Some vendors do better than others because they handle better the larger buffers with their drivers than others.
MT at high speed is relative weak. On 400Mbps PHY (40 MHz channel, 2 chains) then 260 Mbps is the max data rate to get. On 867Mbps PHY (80MHz channel, 2 chains) MT only gets 360Mbps as max data rate. Bi-directional will halve the data rate.
Things have improved with the "wifiwave2" drivers in RouterOS 7, on the devices that support it (limited list today! e.g. hAP ac3, Audience, RB4011 ...) many MT devices are NOT in the list ! Data rate is reported as 500Mbps+, like other brands.
Very extensive information on this can be found here:
https://www.duckware.com/tech/wifi-in-the-us.html
And maybe this also:
viewtopic.php?t=182503