I'm curious to see what this would look like as well. Because waxing the bumper is a bitch when you have to deal with that black plastic. You don't want to get wax on it!
The flip side though is with the California Special Front, keeping the back difuser black makes it look symetrical. So I'm not sure if it would be a good idea to paint it if you have the CS Front also. Unless of course you plan on painting it as well.