I had an interesting discussion with a fellow gearhead, and now I am confronted with a bit of my own ignorance. Having bought a 'new' used car, I am doing my standard *Replace all the fluids, Inspect for dodgy rubber bits (phrasing!?) and Refresh ignition system" dance. He suggested to replace the old ignition coil with a new one for 'full output', and it was likely 'worn out'. I recall my father suggesting I replace my coil on my 60s Beetle for more power, and it seemed to work, but may have just been a more modern design.
Now, part of me accepted this at face value. Perhaps electron stripping, heat cycles, vibration, and degradation/fouling of insulators will rob horses (thats a hangin!) But the electronics guy in me is like, "It is a coil of copper wire, a simple transformer, either it breaks (from overheating from worn plugs, vibration, etc) or its working to spec." I.e., if resistance is fine, leave it alone.
Anyone have any input? Maybe someone has a new and used coil of the same batch they can measure for resistance? Any first hand knowledge or better yet, hard data? Armchair free-association welcome if on-topic.
But, I've also seen a lot of coils fail due to insulation, such that the secondary winding is arcing right back to the primary through the case, or outside the case to the ground it's bolted to. So the windings are not the only thing that can fail. But, if it's working it's probably working properly. The only reason i would ever preemptively replace a coil is to prevent me having to do it at a less convenient time later. Even then, that only feels like a good option when you know the new part is high quality. The internet is now flooded with incredibly cheap ignition coils and a lot of tales of short lifespan to go with them. 
The coil on plug units seem to vary by manufacturer. I don't know whether I've ever changed a GM LS coil, but I have done lots of Ford, Volvo and Nissan coils.
