So you've stated the benefits of having a reliable spark from good leads and plugs (no misfiring), but what does a 'better' spark do? As far as I'm aware, a strong spark can be obtained from a 99p spark plug, just as much as an all singing all dancing Iridium one could.
I chose Iridium because of the supposed longevity that, I am lead to believe, is the difference between the 'premium' and the 'value' plug (although Peter @ GSR tells me that may need replacing soon, having been in for 10 months). I am also aware that modern engines are built around Iridium plugs, why or how I don't know. I would imagine the longer service intervals modern cars have now are a factor.
Just out of interest (and with due respect), do you know what a flame kernel is? From my limited web trawling of the past on the matter, spark plugs have nothing to do with their duration or size (I'm guessing that would be more down to mixture and timing), I understand them to be the start of the combustion process in the cylinder.
The spark plug initiates this kernel and, of course, a nice strong spark will no doubt reduce the chance of the kernel 'dying' and causing a misfire, but this brings me back to my original point. Mr 99p plug could do this, albeit supposedly not for as long as Mr Energiser plug.
Even the longevity credentials are questionable. I'm only looking at what I hear from the makers themselves, the public and my own logic. And we all know that the makers are trying to sell their product. Only recently, the makers of Innocent Smoothies were in hot water for advertising false health benefits. Don't believe the hype that's sprouted from the horse's mouth; marketing get paid a healthy (ahem) salary for a reason.
There are many sides to a story, however. There are people who have running troubles with a particular type of spark plug, so maybe there is a difference somewhere. I know that the Bosch "super 4" plug is supposed to be bad due to the four terminals hindering the flame kernel's development. After all, the spark will only occur where there is least resistance, meaning only one terminal will be used at any one time anyway. I used these in my old, stock, Punto 60S; could I feel any difference (for better or for worse) after these were installed, plus an oil change? No. Would there have been any difference on the rolling road, if I had put it on one? I very much doubt it.
If you, or anyone here, know more on how these processes work, it would be great to hear further explanation/correction. I'm open minded on the matter. Also, if you got this far then wow, you must be as bored as me