yes, EnableJournaledDiskSyncs=true in the KahaDB persistence adapter is the same as syncOnWrite=true in the AMQ persistence adapter.
The default behavior is to sync (fsync) on a write, to comply with the jms durability requirements of a synchronous persistent message send. A message must be on disk before the brokers reply.
When set to false, there will be no 'file sync to disk' call from the persistence adapter (fd.sync or channel.force). It is then up to the OS syncing writes to disk in its own time, so there may be a delay depending on filesystem buffering. In essence this means that there is the possibility of missing messages (from a client perspective) in the event of a failure.