Late to the show but the lateral forces on the pipe would require additional bracing. With the pipe offset from the transom outlet, there will be rotational forces at the flange. One good flange at the top of the pipe directly back to the transom would eliminate most of the torque on the flange. Most of the time you would never see it as the swim platform would be covering it. The flange would have to be stout as it would hopefully take the brunt of the stresses that the flange was taking.