http://forum.mikrotik.com/t/single-router-wifi-coverage-ac2-vs-ac3-vs-audience/150185/1
The TX-power can be limited by the EIRP regulation. It is more the case in Europe (ETSI / 20dBm) than in USA/Canada (FCC / 30dBm)
The real “gain” in coverage comes from the fact that the antenna amplifies the received signal (just as it does with the sent signal), and makes the receiving more sensitive. You “hear” the client from further away.
Also may also want to read this about the hAP Lite: http://forum.mikrotik.com/t/hap-lite-as-an-access-point/155253/1