There is absolutely no doubt that heavier cyclists go faster downhill than light ones. I'm a heavy guy, and when I do the ride for heart, I coast downhill on the upper sections of the Don Valley Parkway passing waif-like cyclists who are pedaling.

The reason for this is air resistance, the main speed-limiting factor in cycling. The same reason a golf ball flies farther than a ping pong (table tennis) ball. Air resistance is mainly related to cross sectional area, while gravitational force is related to mass. When you coast or pedal downhill you accelerate until you reach a terminal velocity where gravity and any pedalling effort match the air resistance slowing you down, and your speed is maxxed out. If the OP and his bike together weigh 170 lbs, and he adds 20 lbs weight, he has increased the gravitational forces pulling him downhill by 11% but he has almost certainly not increased his cross sectional area by 11%. So he will be noticeably faster downhill. If he were riding in a vaccuum, or had a tail wind that approximately matched his speed, the extra weight would not help.