I did not look at the thread for a few hours and I am now behind the response curve.
Conductivity of springs: already pretty nicely explained in subsequent posts.
In a (sort of) short reprise, the whole volume of the spring wire is conducting current. The coating is mostly to keep spring steel from rusting. The steel is the major conductor. If you had a stainless steel spring wire, no coating needed. But as I recollect, it does not have optimal characteristics for springs, has a somewhat higher resistance, and I believe is harder to solder. But, when I last looked at these things, steel was made using indentured servants working bellows for the charcoal fire which allowed (with additions) the reducing of iron from the ore. So keep a grain of salt handy while reading.
DC current does indeed use the entire volume of the conductor. As the frequencies of the current (alternating current) increase there is a general migration of current (due to skin effect) from the whole volume to the outside of the conductor. But you need to go to significantly higher in frequency than involved here to get the current sitting on the conductor surface.
Phosphor bronze is both a better conductor and less prone to corrosion. Nice and springy as well. Gold, silver, etc. plating would also improve the corrosion resistance, but probably not necessary. The possibility of Phosphor Bronze is interesting. Done in similar cross sections, this would improve current to the bright bits. I am guessing that a PB spring replacement for the main spring and elimination of the second spring would still yield a higher delivered power to the LEDs. In this development the emphases has been on current, but lowering the resistance in the spring would up the available current (within the constraints of the regulator electronics) and also increase the voltage across all the light making parts a bit. That might be good or bad.
Now a disclosure: I don’t care how bright the light can get. I am one of the GA users. I would have liked to have the tube and stuff able to use Elon’s batteries in the future. Perhaps a nice project for us to do the Q8 in nextgen battery formats. Maybe only a tube needs to be changed. I want it to run as long as possible at fairly low light outputs. In an emergency, I am unlikely to try to use a flashlight to start a (forest) fire, see if I can melt a battery, or to heat survival food on the head of the light. So battery POWER required to be delivered to the LEDs for a given light level is more important to me than simply more amps. The more voltage drops in the power train (with constant load current), the more power is being consumed from the battery for the same amount of emitted light. Power is, in one form, VxI. If, for the same consumed current, the voltage supplied by the battery can be higher, the battery is operating at a level which is consuming less overall power (the heat generated in the resistances in the power train). Hence, battery life is a bit better.
Oh, protected GAs need to work.
So, for any given LED output, less wasted POWER is required from the battery with lower resistance in the total circuit. May not be a significant value; but it is there.
Oh, I am not fond of solder blobs. They are relatively soft (at supportable application temperatures) and can wear in normal use. This seems like a bad idea. Particularly for a use that involves sliding blobby battery posts against Copper rings, when screwing together parts in the battery tube, for normal use. Clearly there are many that find no problems with this. Solder can form an oxide (with today’s air; also various sulfates, etc.) and cause a voltage drop. Plus the aforementioned pressure and friction. Just don’t like the idea.