Really, really good observations, BUT, you missed out on two things:
David.- Otto has probably made more for the understanding of the piston re-bound than anyone else I know, including the Cardews. His Dead-Blow piston works, and proved itself well. It is a much more elegant solution than the Cardews' (or any other, ABD -AntiBounceDevice-), so Otto could talk a thing or two about piston efficiency if he cared to.
You also forget that tests have proven undeniably that GasRams are LESS efficient than spring driven guns. I.E. they need more stored energy than spring guns to yield a given power output. Where´s the trick? In the peak force required to compress the Ram. Why? Because seals are inherently inefficient and Spring guns have ONE seal, but Gas Rams have at least THREE. AAMoF, it's the seal the one that is most responsible for losses (not his fault though, he does the job he is commanded to do), as it is the part the creates all the friction. A smaller, thinner, better seal is much to be preferred.
The problem with GasRams and Diana guns is that the piston locks in the stem. Not like the British versions where the trigger locks into the piston wall or into a shoe. Since the piston locks in the stem in the Dianas, the piston starts it's race into the cylinder perfectly aligned, not crooked. That is why Dianas are better piston guns than other guns. BUT it also precludes the creation of a GasRam for a Diana. Only Vortek did one many years ago and it was a dismall failure. It worked well for a few thousand shots and then broke down. I know, I had two and probably the only extant working piece is in my collection. So, until we can find a way to improve on the Vortek GasRam, we will need to stick to the Steel spring.
Otto.- more than thinking along the tapered wire spring, you could also think along the variable pitch spring. That will also put less mass where you want. BUT you need to remember that either way, there will be more stress in the sparsely populated regions than in the closely wound ones. And Stress, coupled with Fatigue, is what kills a spring.
And again, I repeat, when you use heavy for caliber pellets, For the spring it feels like taking multiple shots per pellet delivered. The spring encounters that suddenly-braking driver and decelerates suddenly, but the energy has to go somewhere and it flows BACK into the spring, re-loading it. It then goes forward to a piston that very little room to manoeuver and so, rebounds again. This goes on about 5 or 6 times till the spring's guide can dampen the whole dance.
So, shooting ONE heavy for caliber pellet feels, TO THE SPRING, as shooting 4, 5 or 6 times, therefore, fatigue and stress take their toll.