The last analysis made by AOML just prior to landfall suggests Rita was right on the border between cat 2 and 3. Guess TPC went on the conservative side and stuck with the 3. Whether cat 3 conditions existed somewhere along the coast or not we'll never know - but it doesn't sound like too much of a stretch based on what observations there are. Expect it will go in the books as cat 3 at landfall.
I expect it will go in the books as a cat 3 as well. Since you don't have observations from every area this storm hit it would be hard to prove that cat 3 winds didn't exist somewhere. Unless it was clearly no where near that strong. On the other hand, if you found evidence of a cat 4 then it would be upgraded (I'm not saying that evidence exists).