With directional antennas, the client usually can see the signal from high distance. However, sometimes the clients can not connect if they are far away from the AP although they can see the WLAN. The reason is that the access point has enough power to transmit for high distance while the client adapter does not have enough power to trasmit the signal to reach the AP. in this case there is some kind of one-way communication.
If your area is small and your APs are not too far away from the client, you can use directional antennas if you make sure that directional antennas will cover the necessary area.
I have no practical experience with directional antennas so I am not able your second part if the coverage will be better or not.
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