I think the first boat to use side exhaust was Ted Saboreese 47 Fountain INXS. The only reason for side exhaust is to project more sound toward the crowd on the beach. It only works on a full staggered setup and it really opens up the engine bay around the rear motor.
Here is a shot inside of The Muscle Super V when it ran blower motors. Notice the side exhast.
Not sure if any builder other than Fountain has ever done this.