1. That the Pertronix coil does not require the use of a ballast resister is not the point. The point is: Does the engine react well to one being added.
2. That does not mean you HAVE 'full voltage' at the coil.
3. The question is not, 'What is the temperature of your coil?' The question is, 'Is your coil overheating?'
A sensible, reasonable way to determine if your coil is overheating is to replace it, see if the symptoms change.
Any auto ignition coil would do. One from a junkyard (that meters out well), one from some other car that has one.