E-3 plugs worth the money?

If your car runs better after you replaced the plugs it's because you needed new plugs not fancy plugs. The arc (spark) will follow the path of least resistance, generally the shortest distance. A clean sharp edge has a big effect on arc propagation, this is where any fresh plug shows a difference. Proper heat range is huge for plug life regardless of brand. Generally 2-3 ranges colder than the old leaded fuel spec is just fine.