Ballast resisters

Within reason, the value of the ballast resistor probably only affects coil life. If a coil specifies a certain value, that is the one you should use. The purpose of the ballast resistor is to limit the charging current into the coil. The coils are designed with this resistor in mind. the operating voltage on the coil is designed to be less than the full battery voltage when a resistor is used. As with many things, it will work ok over a range of values.

If you have a lower value ballast resistor, there will be more current in the coil. More current can give a hotter spark, but it also translates into more energy consumed by the coil when it is not firing. More energy makes more heat, which shortens coil life.

More current in the coil does not necessarily linearly increase spark output. The output voltage at the coil is largely determined by how fast the ECU breaks the primary circuit (like points breaking the current flow) and the inductance of the secondary circuit ( coil wires, return through the engine block. Higher quality ECUs switch faster and make hotter sparks. The inductance is almost completely dominated by the loop area enclosed by the secondary circuit. This is the path from the coil through the wires, plugs, and return back to tha coil case.

You can think of loop area like a circle. If the wires in the circuit are far apart, there is a lot of area inside the circle. If the wires are close together, it is like squashing the circle flat, not much area inside the circle now. The smaller the loop area, the higher the voltage.

Low inductance is why coils mounted directly on spark plugs are much smaller and work as well or better. There is almost no loop area, so you can get a hotter spark with a lot less primary energy. Remember, peak spark voltage is a strong function of the loop area of the circuit.

If you have your coil mounted off the engine, you should probably have a big honking (technical term) ground wire from the coil to the block, not only to carry the primary current, but also make a short path back to the coil for the spark plug current.

With all that said, the value of the ballast resistor was specified by the coil designer to provide the maximum amount of spark, with a minimum amount of energy wasted to just heat up the coil.

Hope this makes sense.