XLR cables are always balanced (+ - G) via the 3 pins. Since the cable is unbalanced it's actually operating at a different impedance than a standard XLR (-10dB unbalanced instead of +4dB balanced). Unbalanced cables only have a +hot and -hot, no ground, which means that RF noise, cycle hums and ground loops are not eliminated resulting in noisy signal. That's just some extreme basics on cables.
But now on to the issue, I'm NOT FAMILIAR with the circuitry of the Neumann system because I've never seen a schematic or signal flow diagram of the system itself. The fact that the cable itself is unbalanced doesn't matter if there is a balancing transformer between the cable and the final output of the signal. If the final output from the mic is a standard mic level signal and not overly quiet (14dB quieter aka nearly 5 times quieter) than you can bet the signal is being balanced somewhere in the path. An unbalanced output would be very quiet and noisy (the same as an instrument output without preamplification) so I'm willing to bet that the system was designed to balance itself. HOWEVER, if the output is unbalanced you can just buy a direct box or line isolator to balance the cable which will output a clean and balanced signal.
Could you replace the stock wire with silver wire? Yes. You would need to reverse engineer the cable and build a replacement with the wire of your choice. The active cables have a different number of conductors than standard mic cable so you would need to have someone construct an identical cable with the silver wire. Someone who really knows that they're doing would be able to do this but the labor involved would make is more expensive than it would be worth. The results would be theoretically more pristine, due to silvers higher conduction efficiency, but since the cable itself is only passing signal from the capsule to the body without involving any circuitry or processing the audible gains in clarity would be negligible. I'd be willing to bet that a blind side by side would sound identical to human ears without spending many hours familiarizing your ears to the slight difference in the modified microphones sound.