Is your hatchback A3 or S3? The S3 exhaust is much thicker than the A3 one, perhaps double as thick and there is not enough place under an A3 trunk for a S3 back box. You could modify the mounting brackets, but the muffler would hang closer to the ground.
As to the pipes, I suppose that if you cut the Saloon pipes, you would have to bend them slightly outwards in order they to fit the cutouts in a diffuser.
Mind me asking why do you need it?