Seeing as how there are subtle differences in the actual firing voltages and therefore the effective cylinder timing between types of plugs, I suspect any matched set of plugs, from $1 loss leaders to E3 would have made an improvement. Not exactly a glowing testimonial.
I like copper core, except you have have to change em more often autolites worked great for me, the idle was MUCH smoother than stock, saw my gas mileage rise a bit, when I pressed the gas I had a better response but that my experiance.
yesh coppers cores are better for performance has they dont retain heat like precious metals do, I can always switch to a set of 103's if i need to which I'm sure all you know is one heat range cooler then whats supposed to be in there.
E3s are a gimic. The volts will always pass thru the path of least resistance evey time. ie one pathway. The others are never used until the resistance in one increases. Then it will move onto the next one and so forth. The heads are purposely built with one spark path on mind. A controlled flame front originating from one location/ source.