Voltage drop across ballest resistor

The ballast resistor is nichrome wire. A fancy name for nickel chromium, or stainless. They get hot, but don't seem to burn paint. Based on that, and tables showing nichrome resistance vs temperature estimates the change might be 10 to 15%.

There is resistance in coil primary too, add in the ballast, the supply voltage divided by that, is the maximum current. Coil current ratings are not often specified, but 4-7 Amps is typical.

Coils only take a few milliseconds to charge, after that energy going in is wasted in resistance. At low RPM there is a huge amount of waste.

Modern ignitions use time based predictive dwell control, eliminating ballast, saving significant energy, keeping coils and transistors cool.