Here is the conclusion slide from Zhang's 24 Aug 05 presentation. They note that the GPS profile improved not only the forecast track but the estimates of low level moisture, surface pressure, and "cyclonic vorticity," all of which are critical to accurate hurricane forecasting. This is presumably due to the unique penetration of the occultation deep into the storm's interior, which otherwise cannot be observed. Zhang's presentation and others given at the NOAA workshop can be obtained at GPSRO Workshop.