Hmm... Are you sure it's not connected to a "Fan-only" 4-pin molex? Some PSUs have those which are designed to undervolt fans and run them at slow speed.
Anyway, if your fan is connected to a regular 4-pin molex that you would otherwise use for an optical drive or HD, then yes it's running at its top designed speed. In that case (lol, pun intended), you might want to buy a faster-spinning exhaust fan... Though I think a better idea might be to dremel out that stupid stamped grill (which is too restrictive to airflow IMO).
HTH.