I don't know about power improvement or mileage gains on the E3's, but from my experience they do help my vehicles start faster and are harder to foul. I was having starting problems with my Grand Am. The engine took around 5 seconds of cranking to start up. It had Delco's installed so I replaced them with E3's with high hopes. They were given to me so I figured I would give it a try. After the swap the engine started right up in under a second.
I figured maybe the improvement was because of the new plugs in general so I swapped them out for brand new Delco's. The problem came back. I put the E3's back in and it runs fine again.