There's been lot of debates about this topic earlier and it is an interesting one for sure. A bad news for those who expect now a DEFINITIVE answer: I can't deliver that.
We did some brewing with different piston gaskets some days ago. I made a few
shots (30-35) in quick succession.
It was bit of a challenge changing the gaskets as fast as possible (releasing steam, pulling out a piping hot piston unit) but a huge beer jar came handy for cooling the piston unit and making the gasket change easier.
Water inlets (still under pressure)
It was interesting but the differences in yields and the observed pressure profiles showed only subtle differences. Except of this the original gaskets of the Pro800 weren't new which means it wouldn't be fair to reveal more about that.
The next day two friends came by with another Pro800. We removed the inner spring of the group of the machine in the roastery. A perfect setup for a comparison of two otherwise identical machines side by side (same pid setting of course). That's the video we made of the simultaneously pulled doubles:
As said, same grind/dose - 18,5g, temperature - pid 120C, same tamping force but the machine on the left without the inner spring, on the right with a double spring piston unit. As result different pressure profile (of course) peaks at 7 respectively 9 bar, different taste (no surprise), but same yield (45g), same pour (55 sec 10 sec pre infusion included).
It was interesting tastewise too. We pulled Ethiopean Jimma Seca and another farm selected coffee from El Salvador (both of them medium roasted) and a 100% arabica, dark roasted Belarbar blend (from a fellow roaster, Benvenuto Belardi in Umbria/Italy). All three of us preferred the dark roast with the single spring version but we were divided with the medium roasts. Making the results even more confusing I preferred the Ethiopean with the single spring setup and the coffee from El Salvador pulled with the dual spring group. I think there is no black and white in this debate but it was fun to make and very interesting to see the different pressure profiles BUT identical pours and same yields as result.