This Astra Turbo (Powered by Cosworth) needed a pair of heafty exhaust pipes. Rather than have a large pipe hanging below the rear bumper it has a pair of exhausts cut into the rear bumper itself. You must always leave a bit of clearance around the exhaust otherwise you will get a lot of vibration and noise and the heat can deform or mark the bumper. If heat is a problem you can put a metal plate over the bumper to protect it.
Having exhausts exiting through the bumper gives a little more ground clearance and will avoid the problem of scraping the back box on speed bumps when a car has been lowered.