1. You can paint the muffler with
flat-black header paint.
2. You can build a sheet mental cover (fiberglass won't survive that close to the muffler) and paint it flat black, but it will probably trap too much heat and direct some of that heat to the trunk area. Remember, the battery and emergency battery disconnect unit is directly above the muffler. BMW left it exposed to allow the heat to escape.
3. You can remove the muffler and substitute a small diameter straight pipe that does not extend below the bumper cover, but you, and your local police might not appreciate the noise.
4. You can avoid looking at the back of the car from low angles.
