You can polish it but just don't expect it to look chrome or shiny, although it will look cleaner.
I cleaned the entire back box and tip (yea sad I know) using Chemical Guys orange degreaser, then autosol with a wire brush. Box came up like new and people were asking if I just had a new exhaust put on, so it does notice. The tip however didn't come up so clean and silver, but looked better. Worth the effort, then just keep it clean as you go.