By lowering the number of SSIDs you are broadcasting you can reduce the amount of airtime used up by beacons. This depends on your minimum data rates, the number of SSIDs, and the number of radios on the same channel.
Revolution wifi blog as a great excel calculator which you can use to find out how much overhead your network configuration will result in.
http://www.revolutionwifi.net/revolutionwifi/p/ssid-overhead-calculator.html
As for antenna being high in ceiling, it really depends on the AP and antenna you are using. Its hard to answer without seeing floor plans and knowing more about your environment and infrastructure. Typically keeping AP/Antenna above the users is best as it results in better RF line of sight to the device, but it's not always the best bet.
_ELiasz