I don't know how much of what I've heard over the years is true, but here goes.
Factory installed 'Best'... same exact tire, Harley branded 'Better'... same exact tire, Dunlop branded 'Good'.
The only real difference I ever noticed was changing from Factory to aftermarket... and even then only by a couple thousand miles.
I switched to the Metzler M880's as aside from handling, didn't get that much of improved mileage.
I switched to the Michelin Commander 2, and not only gained improved handling, but have gained, IMO, significant mileage.
On these 'hard-on-tires' roads in Alaska... and my riding habits, I've averaged 7,000 to 9,000 on the Factories and Metzlers. Since I installed the Commander 2s, I'm currently sitting at 9,000 and have about another 2,000 or so to go before I hit the magic Lincoln Head.
Not a significant number for some, but if I was in the Lower 48, my mileage would be close to some of yours... in the 14,000 to 18,000 range I would hope. I do remember getting 26,000 out of a set of Dunlop GT Qualifiers on my little Kawi 750 back in the day, but those days a long gone.