A misfire is still a misfire, and a misfire is what is caused by a plug wire with internal breaks. It can be part time, but it is still a misfire and is very obvious, like i said.
A weak speak causes a misfire under load at low RPM, that weak spark can be caused by a high resistance plug wire, or other factors. However, it still either causes combustion or doesn't cause combustion.
What you mention, checking plug wires with an ohm meter and/or flexing them is first year stuff, it's not exactly a secret. I was not talking about how you find the source of a misfire (random or not), the diagnostic process isn't relevant here. Nor is the very true fact that you can have a misfire in some RPM bands and not others. The point, is that plug wires that are not causing a misfire do not effect fuel mileage significantly, something you seem to have missed.
The ECU's #1 priority is the mixture, all other things are secondary, and it's the mixture that primarily controls fuel mileage, assuming there aren't failures elsewhere.
I'm glad you're certified in heating/cooling/AC, but i don't see where that gives you any special knowledge in ignition systems of internal combustion. It means that you're a passable automotive HVAC guy, which is great for HVAC stuff.