Talked with Cummins Care about all these subjects and they said it is possible the key ignition switch can affect the "switched" voltage to the ECM.
And they said, if your voltage is degraded somewhat, but not so bad that it would flash a fault code, then your engine performance could be affected, because a degraded voltage to the ECM would produce possible negative effects. This include idle, acceleration/performance, and even fuel efficiency (MPG).
FACTS: You can't tell if you have a degraded ECM condition since your engine will start and run. And things like engine sound at idle is rather subjective anyway, but most people can feel the positive effects on acceleration and MPG, which are real.
Cummins Care added that a loose or bad ground from an either an "unswitched" or "switched" voltage to the ECM can have similar affects and/or consequences.
Of course, looking for a bad ground can be very time consuming. But there are some obvious places to look; i.e., wherever you have an ECM ground or 12V power line held down by a bolt and/or nut... that might have vibrated loose.
For example, this might include inspecting the buss bar located behind or near your fuse box area. Just wiggle these connectors and tighten any loose bolt/nut you find.
Here's a YouTube video on this subject you might find interesting regarding an Unswitched Low Voltage (Fault Code 434).
Considering the cost of replacing the key ignition switch ($20-$30); the possibility of this part failing after 10 years of service; and the how easy it is to replace; and the upside of improved performance... I'm very glad I did replace it and I wish I replaced the ignition switch 3 years ago or 20,000 miles sooner!!! This preventive maintenance tip would have saved me a lot of aggravation and money in fuel savings too!
Yes I started this story off by talking about bad fuel. However, I did change my fuel filters out after that... drove for 300 miles on new filters... and then decided to replace my key ignition switch as a preventive measure.
So I should have never mentioned the bad fuel problem while talking about replacing my key ignition switch as I see these as two separate concerns.
In my case, one problem (bad fuel) which forced me to get service; lead me to find two (inactive) fault codes flashed; and that got me thinking about replacing my ignition switch.
...And after I changed the ignition switch I immediately noticed a smoother engine idle sound, better acceleration and MPG.