Need_O2, to reduce droppers at game start you need to inform people when the map will start.
For example if someone just finished downloading, count down from 5 to give that player time to leave. Equally never start as soon as someone joins, give them 5 seconds to decide if they are staying or not. Although slower, these methods will reduce the number of starts having to be aborted due to droppers by a noticable ammount.