I am a firm believer in copying what works. Everyone (repeat everyone) that has ever wanted a model to go fast (except as mandated by rules) has always had a rounded fuselage, streamlined everything, cowled engine, airplane. In the land of the jets the shapes have changed, flatter fuselages to hold two engines, etc. but given a choice it would be the upright oval that was previously mentioned. If the fuselage has any angle of attack it will smooth out the flow around it and have a lower drag. The box fuselage in a high angle of attack, high speed turn is just drag in one of its best (in terms of being worst) forms. At least with drag due to lift you are getting something.