It looks that it will be easy to replace 325/328 exhaust with 335 exhaust or a dual/quad exhaust because of the new muffler design. All the previous 3 series except 335/M3 had an storage area in the right side of the muffler under the rear compartment area which is not like the F30 as we can see in the spy photos. Also the rear diffuser is a separate part from the bumper which means there's will be no need to change the whole bumper when fitting dual side exhaust.
Also rear view camera can be seen in the following picture