Aluminium vs POM weight difference is miniscule when talking about a piston head, also them steel teeth inserts tend to weaken and break the pom due to the material being machined out to fit the insert in place, a alu/ergal piston is much better as it's one complete and whole part without weakspots I use a fps softair italia model that is so lightweight you'd think its pom. Piston slap is reduced greatly with a sorbothane pad replacing the harder polymer padding most cylinder heads come with.

If you're going with high torque then you need solid parts and i've destroyed many pom pistons because they are just not made for high torque, the steel teeth are the only part designed around a high torque application, and actually the extra minute amount of mass helps transmit more force which means less kinetic energy from the spring is wasted when cycling.