All the discussion I have read so far indicates forecasters do not expect the kind of strengthening Katrina went through. Apparently she stirred up cooler water so the fuel will not be there.
3 weeks have passed- plenty of time for the Gulf to re-heat.
Also- this storm is not taking the same track as Katrina so will be passing over waters not touched by her.
Read an article yesterday about pockets of sea water that extends to 3,000 feet below the surface. The temps in these pockets are as high as 90 degrees and when the storm hits these areas a quick drop in pressure produces an increase in winds and rain.