FYI in the next nightly build you should find AD sync + auth is working and supports differential sync (only pull in changes since last sync) when a new user is successfully authenticated.
We've also created a new authentication subsystem type called ldap-ad that has some more useful defaults preconfigured for Active Directory.
We found that if you use a userNameFormat that matches the userPrincipalName (UPN) of your users (these seem to be <sAMAccountName>@<domain.dns>) you can get authentication and sync working in tandem
ldap.authentication.userNameFormat=%s@domain.dns
ldap.authentication.java.naming.security.authentication=simple
ldap.synchronization.java.naming.security.principal=alfresco@domain.dns
ldap.synchronization.userIdAttributeName=sAMAccountName
use DIGEST-MD5 instead of simple if your user passwords are stored with reversible encryption, but this is not the default and passwords would have to be reset.
Alternatively, chain the passthru subsystem so that authentication is performed more securely.