Hi David
I am just looking at a new coil, and think I will go for the Lucas Sport coil, which is a full 12V coil - I have a 75 TR6 with a ballast resistor.
As I understand it, the ballast resistor is in series with the power line to the coil, and drops the voltage to something less than 12v - I think it is supposed to be 6V but I have read 9V somewhere. When you operate the starter, the ballast wire is bypassed at the relay and the coil is fed at 12V to give a hotter spark to help with cold starting an emissions engine.
The simplest way to bypass the ballast resistor I have seen is to move the white/yellow wire from the starter relay to the fusebox connection at the white wire (upstream of the fuse, if you get my gist). This means that the ballast resistor is essentially always bypassed as though the starter is turning.
I have no idea whether the sports coil will make things better for my engine - I plan to keep points but use better ignition leads since I feel this gives me scope to improve performance in other ways later without having to worry that I will be overtaxing the ignition. I could be wrong, and maybe I just like the idea of a "sports" coil and fancy red wires!
Hope that is of some use
Alistair