Oooooh....that's a big, big, sweeping question, Petey Boy!

Are you asking would fitting a heavier piston than optimal in a particular gun would rob power if converting? Maybe. Maybe not. As AB says, it'd certainly give a different feel to the cycle. If the spring wasn't changed, piston acceleration would be slower. But momentum would be increased. Recoil in the first forward stroke would fractionally increase, but piston bounce might decrease. Transfer port size and flow and pellet release pressure would also be big factors here. Depending on how optimal the piston weight was pre-fettle would dictate whether a lighter or heavier piston would be an improvement or otherwise. And it's often subjective and psychological anyway.

A heavier piston combined with more spring force and more preload could well result in greater power output but at a detriment to the firing cycle, usability and accuracy.

Depends on the gun.......I've often heard (but haven't experimented with this) that increasing the piston weight of rodless 25mm pistons (as fitted to Gamos and clones) might be helpful. But I quite like the quick feel of their cycles with their light pistons.

So much to consider here.....