I do way to much reading - At a decent speed, air flow under the car can restrict the exhaust flow on turn downs and even normal flush exit pipes, this is why many exotics and super cars have the exhaust pointing up a bit at the back, also helps the aero at the back by filling the low pressure area with pressurized exhaust

racerx - check thumperdarts real world weight loss and costs thread, FMJ has some numbers on exhaust weights hiding in that 5 million post thread