Currently the implementation for the FIM Portal is essentially a listener, although it won't use a listener operation. However, it does not benefit from the checkoperation/listen operation pairing that AD/LDAP listen do.
If changes were missed for the FIM Portal listener, you'd have to wait for subsequent changes or the synchronization oplist to fire.
Consider adding a polling schedule for explicit delta-import delta syncs:
Benefits: Don't rely on synchronization oplist
Demerits: Scheduled delta imports.
Customer support service by UserEcho