Well I just had another little go with my LP53 and it seems I've confirmed your findings. My added piston weights come to exactly 18 grams and shoots 7.56gr H&N Finale Match wadcutters at an average of 465fps. Adding 2 grams, just barely possible with the space available, dropped velocity to 435fps. Removing all the weight drops velocity to 370fps or so. Going up to 16 grams brings it up to 445fps. Adding a 1mm shim under the spring guide (even 1.2mm hits coil bind before the sear clicks into place) drops velocity to around 455, but becomes erratic, jumping between high 430's and low 460's. So 18 grams added weight and 'just right' preload (about 6mm) from the base of the spring guide are just right for power.