A heavier piston does not always mean a higher velocity will be obtained unless detonation occurs and in some cases a heavier piston will reduce the velocity and at the same time create a harsher shot cycle. What does happen is the heavier piston hits the bottom of the cylinder harder and the cushion of air does not absorb some of the shock of the piston hitting the bottom of the chamber. Also, the weight of the piston increases the compression ratio and can cause unwanted premature detonation rather than combustion desired, especially if excessive fuel comes into play.
Also, higher velocity does not necessarily mean a harsher shot cycle. If properly tuned, many guns can have a higher velocity and will also have a far better firing cycle. Both the loss and the improvement of the velocity and firing cycle depend on several factors including the seal and chamber sealing properties, the lubes used where, how and how much, the spring and guide as well as tophat fit. Also, what method used in reducing the torque on what gun impacts the firing cycle.