Our good friend Hogasm here will tell you that Harley sells good parts for the most part, I tend to agree, for the most part.
The SE plugs have done well for me. Do I need them over stock? Probably not, Do they last longer? I don't know. But why not put SE plugs in my SE

My bike has 35k on it now and currently has it's 3rd set of plugs. And I did not change them because of a problem. I just decided the first time to put the SE plugs in and the second time was only because it was discount night at the local dealer. And I have a new set of SE's on the bench to put in with my current projects because -- it was discount night.
The plug change interval on my last truck was 100,000 miles! I traded it in with 85k and the original plugs - no problems.
Would I put new plugs in if I was going to the dyno? No - I would put new plugs in when I got to the dyno shop though.
Running the bike on a dyno will show the difference between plugs.
But what are you doing on a dyno? You are loading up the bike to its max trying to wring every little bit of horsepower and torque out of it. You want the best plugs in it then.
Be honest with yourself. How many times do you actually run through all of the gears, shifting hard at 6000 rpm? Not just 1st, 2nd and 3rd, but where you really start to put a load on the engine - 4th maybe even 5th? If your running that hard every day then maybe you need the best plugs money can buy.
And if you are doing it on the street every day then you would be better off investing in insurance and attorney retainers then spark plugs.
Buy name brand plugs from a reputable place, no flea market counterfeit stuff please, and you should be good to go.
3-4 MPG increase with a particular brand - type of plug? Sounds like something else was the culprit that changing to any quality plug would have shown the increase.
Next your going to try and convince yourself that indexing your plugs on your 800+ pound dresser makes it run better on the way to work
