I've driven the Hana Highway a number of times and the rental car "warning" about taking the shortcut back from Hana has prevented me from taking this route.
Last month we were over in Maui with two other couples and decided to hire a driver for the Hana trip. He had planned to take the shortcut back to Lahaina but the day we did the trip tha Hana area had over 8 1/2 inches of rain due to a tropical storm. The local police closed the gravel road due to mudslides so I guess we'll have to wait for another time to experience this.