Back when I was just a lad, "megaphone" exhausts enjoyed a brief period of popularity in snomobile racing. Overall peak power was not as high as when a reversed cone and stinger is added, but there is an advantage in that scavenging is improved over a fairly broad range. A tuned pipe making power at 7500 is less than worthless at 5500, while a megaphone will show measurable gains from off-idle, on.
They fell out of favor with the advent of multi-cylinder engines. The advantage of big bores and strokes is torque at all moderate speeds. In a sense, that megaphone exhaust may be just the ticket for those old contras, because they enhance their torquey aspects over a broad range...they won't "fall off the pipe" as easily.