Youre right, storm surges for a Cat 5 are usually above 18 ft.
Correct, Camille came in with a 25 ft storm surge and waves 25 feet on top of that... amazing. She was also a "tightly" wrapped storm. The pressure was in the upper 800's and dropping when she entered Mississippi.
Bathymetry plays a big part in storm surge, as well. You've got to have a lot of water available for those strong winds to blow it onshore to get storm surges of 20-25 feet.