Have a lot of springers and several Theoben gas rams and believe that piston guns prefer medium weight pellets.
It has a lot to do with the (generally) 10:1 stroke volume and piston bounce. The heavier pellet starts moving later in the shot cycle which is shorter in duration and more abrupt as compared to the longer, smoother pcp cycle. Same is true if you overspring (or pump) a piston gun. It throws off the designed balance between spring, piston, guide, transfer port size etc and can cause more piston bounce which may feel like more power, but is not.
John Whiscombe is adamant about not using heavy weight pellets in his rifles and has convinced me.