Thanks heaps for that.. Its such a black art to me an one could easily be fooled thinking using a "performance" coil or one with high volts or some sales spiel could in fact be worse for your car performance 
I think you are right to be wary of sales talk on coils. There isn't really as much difference between a lot of coils as they would like you to think. Also they want you to think it's a black art, but in fact the essential science is quite easy.
The first thing to understand is that the ballast resistor is NOT there to give the coil more voltage when cranking. It is there to protect the coil and the ECU when not cranking, and it does that by increasing the overall resistance in the circuit (which in turn reduces the amount of current flowing).
Apart from that you mainly just have to understand two things: Firstly that a coil works a bit like a transformer, so the more "primary" voltage you can put into it the more "secondary" voltage you will get out (as spark energy). Secondly that the primary voltage which the coil can "see" will either be 12v (or 13.?v when the alternator is running) or it will be reduced below that if there is a ballast resistor.
The coil's "impedance" is a kind of resistance, like the ballast resistor's resistance, and the equation for the reduced voltage with a ballast resistor is quite easy, like this:
Reduced voltage = 12v x Coil impedance / (coil impedance + ballast resistance)
This is simplifying things a bit but it's close enough for most purposes, so to look at an example, say you have a "performance" coil rated at 40,000v with an impedance of 1.5 ohm and you run it with a 1.2 ohm ballast resistor. The voltage across the coil will be about 7v (with engine running).
The coil was almost certainly rated at 40,000v based on the (artificial) assumption of having 12v across it, so in the example your 40,000v coil will actually deliver more like 7/12 of that, or about 33,000v.
That is one reason why HEI systems can deliver more punch, they run without a ballast resistor so there is a full 12v across the coil.
The other important thing about the resistance is that it slows down the speed at which the coil can charge. Each time it fires a spark the coil takes some time to charge up again for the next one. In the above example the total resistance of the coil and ballast is 2.7 ohm. If it was just 1.5 ohm (no ballast) the coil would receive nearly twice as much current and it could get ready for the next spark much more quickly.
In fact this doesn't make too much difference at low engine speeds when the time between sparks is longer, but as engine speed increases a higher resistance in the circuit will mean the coil having to deliver the next spark before it has got close to a full charge again, and the spark voltage will start to come down.
So that's two reasons why your 1.5 ohm Pertronix coil would give better spark on a HEI system, but it would still not be as good as a lower ohm coil at higher engine speeds, though to be honest you probably wouldn't even notice that.
