I'm at work so I had to read quickly BUT upon quick read I don't think I am quite understanding; I wasn't making the claim that a new coil would be putting out more power,.. except maybe than whatever it's replacing.. rather that all the coils should be putting out the same power, and sort of like you just said the weaker coil is not putting out as much power because of failure within that weaker coil, which is why it's the weaker coil? (So okay may e you could get that put of what I said... I can tell you from personal experience that two Accel. coils and four AC Delco coils on an inline six engine and it won't run right. Some of the coils were probably marginal or wore unevenly, if that makes sense.)
not trying to frustrate you but I don't quite get what you're saying and my experience has been that when you replace them all at once, whether with a set of the same new ones or the same used ones where there wasn't a problem with the coils on a car it came out of, you wouldn't have any codes like you would on a Nissan Maxima which is very picky about coils and the engine ran like it was supposed to without any stumbling or issues. So I do see why you should replace them all at once but I'm not quite getting how replacing just one won't throw off the balance, specifically versus the other coils (which may or may not be nearer to failure than that one.)
To make this as simple as possible:
A coil either works to fire a plug or it doesn't. A spark plug either sparks, or it doesn't. A plug that fires intermittently is one that is misfiring and the cause can be either a weak coil or a bad plug. A dead miss is one that doesn't fire at all. Are we clear so far?
So, there is no "balance" to coils. They are all fed 12V and they are all just triggered by the ECM. If they are of sufficient health to fire the plug properly, there will be absolutely ZERO difference between it and a brand new coil. Still with me?
Like with anything that isn't a wear item, replacing it "just because" isn't being prudent, particularly for things that often times last the life of the vehicle, which coils often are. Now, there are exceptions. If an OEM spec'd a crap part and you KNOW absolutely that they are all going to die, like the CPS sensors on an E39 M5 for example, well, these are isolated cases and should be approached with that knowledge in mind, but for a typical situation with no history or evidence of mass failure, one doesn't just start replacing things that aren't wear items without proof that they are defective.
Now, when you start firing the parts cannon at at something without doing proper troubleshooting, sometimes you don't fix the problem, so you throw more parts at something and then arrive at the conclusion that doing this "fixed it".
It's certainly possible to develop more than one defective coil, but that's still not an argument for replacing all of them, which can be quite expensive on larger engines. I ended up replacing 3x coils over the ownership of our Expedition, my dad has replaced 2x IIRC. That's over 150,000 miles on the remaining 6x coils in his. The troubleshooting and diagnosis was done using Ford Mode 6 data which shows misfire counts for each cylinder and then specifically changing that coil, which is how this is properly approached. Ford's misfire counter is insane and it takes a VERY bad situation for it to register a CEL.
With some OEM's It is quite possible that you may have more than one cylinder showing misfires and replacing the one you thought was defective doesn't solve the problem. This requires clearing the counters and checking it again. Best practice here is to move the coils around and confirm that the misfire tracks with the suspect coil. You can have a series of misfires triggering the counters on multiple cylinders and it has nothing at all to do with the coils and there is an unmetered air leak. Proper troubleshooting is key.
Aftermarket coils can be a total crap shoot too. I did the "Accel performance coil" dance with a couple of my EEC-IV Ford vehicles when I was much younger and two of them left me stranded. Had an MSD coil die on me too so I went back to an OEM Ford coil. Once I did some actual research on EEC-IV, I discovered the OE TFI coils were ridiculously capable and trying to go above and beyond with an aftermarket offering was a complete waste of time and money, there was nothing to be gained, even with an ignition box, which I added to the Lincoln (MSD 6A), and which worked just fine with the stock Ford coil.
) or 5V (on) .. no in between? 
Hot, right?
