you'd be surprised how much material you can remove from a good piston.
From this one, you can even take a smaller bit and go at the spots between those big holes on the top. The vital areas to leave material are at the front near the piston head and at the back of the body for strength at the pickup tooth and closer to the rack. All other areas are pretty fair game from what I've seen of piston swiss cheesing.
There seems to be an unspoken challenge on asm about how much material can be removed from a piston before it blows up.
this one is from an 80rps dsg gun. That's some pretty gnarly swiss cheese. He even ground the teeth off for short stroking AND he cut right trough the rack base after epoxying in the rack.
btw, if you want to drop even more weight, just lose the bearing plates and bearing out for the piston head, put the end right on it and screw it in, you only need the bearing on the spring guide anyways to prevent binding. Taking the bearing and plates out drops a shit ton of weight, you also can adjust FPS lower by removing them. I had to go up 1 spring step to get back to 390ish fps on my p90 when I did that. (guarder sp110 to shs 120) So you can lose about 20-30fps.