The current leaks to ground because the cable insulation isn't good enough; either that or the source voltage is pretty darn high. Quality cable should handle the voltage produced by any high energy ignition system.
The fact that a spark plug cable uses a low resistance conductor (e.g. real wire) in and of itself has no bearing on causing a potential leakage path. The only way radio suppression cables could reduce potential arcing is through their use of a high resistance conductor (non-metallic core) to reduce the voltage applied to the spark plug gap. But that's a little counterproductive in a performance engine.
Joe, I meant to add some clarification to that other thread too.
Sherwood
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