This storm is likely going to be similar to Katrina. It reached Cat 5 over open water, but reduced to Cat 3 by landfall. As most should know, the Category is based solely on windspeed. Despite the downgraded winds and Category, Katrina's size, impact area, and storm surge were all just as large as the Cat 5, just with slightly reduced peak winds. That is what these massive hurricanes do, they get very intense, then very widespread. Even now the discussion I am seeing is about how wide the wind field on this storm is becoming, with hurricane level winds out 40 miles from the eye, and tropical storm force winds out 250 miles from the eye.
Now as opposed to Hurricane Camille, that actually made landfall as a Cat 5, but its destructive path was relatively narrow, mostly confined to the Mississippi Gulf Coast.