Hello Stefan,
Yes, it is recommended that you have unique SSID for each sector, and also use the Preferred AP functionality in the SM to make sure each SM associates with the intended AP. Not doing so could allow SMs to associated with APs that are not ideal and hurt performance.
Regarding SM distances, 300 meters from the tower should not be an issue.
If you have APs co-located in the same tower (which looks like you do) with frequency re-use, you should be running with GPS Synchronization enabled. I would suggest you to read the article posted here: http://community.cambiumnetworks.com/t5/ePMP-Synchronization/ePMP-GPS-Synchronization/m-p/37884#U37884
for recommendations.
Regards,
Luis