OK, gentlemen. I think there are a few misconceptions and misnomers regarding the HT ignition.
First, there is no CDI. CDI=C=capacitor, D=discharge, I=ignition. The little black box is a step up coil that is driven from the magneto. This is a either a marketing ploy or just from ignorance.
A real CDI is much more sophisticated and was designed to discharge a capacitor that is charged to about 400 volts though the coil to step up the spark voltage. It is charged by a transistorized 12 to 400 volt inverter. It requires a 12 v supply. It also causes a longer lasting spark. It is also distinct from a transistor ignition which uses a transistor to switch the 12 volts into the coil.
2nd, Wire: There are a few things that are going on that probably need to be taken into account. Copper and steel wire that is 6" long has less than a few milliohms of resistance. I think performance differences between them will be minute and probably more to do with HV leakage which is a function of the insulation of the whole system and the surface area of the wire in the insulation.
Also, the coils, both the magneto and the step up coil, your black box, will resonate (at some frequency that I haven't measured yet) so you will probably get more than 1 spark out of a spark before all the stored coil energy dissipates. The actual spark current is alternating current (ac). This means that if the resonance is high enough the "skin effect" starts to kick in. That is the tendency for high frequency ac to travel on the surface of the wire and not through the core. This is a big deal in audio and in RF circuits. So the thicker wires will have more surface area for the high voltage to travel on and it's apparent resistance (actually reactance) will be reduced and you'll go faster.
Resistance wire, carbon wire, is designed to minimize this resonance to reduce radio interference. The system is designed to overcome the losses of carbon wire to allow adequate voltage to get to the gap.
This is a relatively low power system. Any moisture on the plug will burn off in a hurry when it heats up but moisture on the black box coil may cause losses. Dirt on the plug and coil will hold moisture and in severe instances will lead to sparks that convert cases and dirt to carbon which is conductive and there goes your spark energy to ground. Use some silicone electrical sealant on the end of the wires that go into the coil and the spark plug boot to seal out moisture. A cracked coil case would be suspicious too.
Spark power is high voltage, 20,000+ (60KV in cars) volts but very low current, microamps, so it is easy for it to dissipate into any moisture.
I've found that the original HT spark plug cap fell apart on me and that there was never any real connection from the wire screw to the connector that clips onto the spark. The back of the little clip was pitted from sparking. I bought an NGK cap from my local motorcycle shop and everything was much better.
So copper or steel wire will perform better than carbon wire. Better insulation will keep the spark current IN the wire. Clean plugs on the outside and clean coils help.
There seems to be a wide variation of oem wire with the HT's from your descriptions.
