Marty, the stock TCI coil is about 2.5Ω. Anything lower could possibly (probably) burn out the power transistor in the box that grounds the coil. Anything higher won't damage the TCI box. Just keep in mind that the higher the resistance the lower the current running through the primary.... with a corresponding lower output in the secondary... in other words, not as hot a spark.
Yes. I run copper core.
Can the TCI XS650 operate with copper core spark plug wires or is suppression core required?
Yes. I run copper core.