I'm spit balling here on this one as I've never actually tried it. With some experience building racing engines, I've long considered reducing pistion weight to increase power and reduce recoil. Here's my thoughts on lightening which are only theoretical. I'll start with what I have done with piston weight and work it backwards.
Its been common practice to make piston weight heavier with steel top hats to reduce piston bounce. This I have proven to be somewhat true, the results vary wildly because less bounce is more efficient and can produce more energy. Accelerating more weight saps power. Often its wash because the sapped power equals any gains from reducing bounce. Typically adding weight increases efficiency more notably with heavier or tighter fitting pellets that are more prone to piston bounce. Adding steel washers above the top hat is one way of tuning weight. The extra preload the washer creates can offset the weight penalty. I've had very mixed results with this. It varies greatly depending on spring and gun design. I've gotten away from this as a practice.
The subject of the OP was lightening. Reducing piston weight and spring weight does reduce recoil. Look at the running 25mm pistons in HW77 and 97s. That's a quick way to make a nicer shooting 77/97. Obviously 25mm Hw30s are easy to shoot while 30 mm R1's are notoriously difficult to shoot. That's an extreme example and piston speed has a lot to do with it but you get the idea. So there's something to reducing weight that reduces inertia and thus recoil. Felt recoil isn't just the movement of weight in one direction. Often pellets that produce more bounce and are less efficient feel less smooth than pellets that produce less bounce. Therefore it's possible that a lighter piston with more bounce may lose some of it's smoothness. I believe one way to reduce bounce is to run more preload. Not necessarily a stronger spring but a lighter longer spring. The compressed pressure will be the same but the extended pressure will be greater and more bounce resistant. Perhaps a lighter piston with a lighter spring with more preload? A lighter power spring also reduces physical weight that may also reduce recoil. There's so many variables the combinations of pistons and springs becomes endless. Then throw in transfer ports which throttle back pressure you can go crazy. Personally I think being more prone to bouncing, lightening will make the gun more pellet picky. Not necessarily a desirable trait.
With my varired experience playing with adding weights I've binned the idea of reducing piston spring assembly weight. I'm also not a competitive shooter. There are tuners in the UK that this is their thing and they got it all figured out. At a hobbiest level it's a crapshoot at best. I'm not saying it's not worth trying. I'm saying is all the variables make theories fun to discuss but you won't know until you do it. If you're serious about this I'd recommend contacting someone like Tony Leech that has a lot of experience with this and purchase a matched set of parts. It'll be expensive initially but the time and money it'll save will likely be worth it in the long run.
