I don't see something as simple as an additive affecting mileage much at all. I could see if helping a bit, but not being any worse than without it. Unless you're buying it by the drum and running it straight.
The only things I could see lowing mileage as far as additives go are: unleaded in the mix, alternate fuels, and large quantities of water.
I don't run additives EVERY tank. Just now and then. I also don't measure it, just dump some in. Reason being, I don't see how 8oz can affect 30 gallons enough to make a difference.
I get the whole needing lubricity, but is there any hard evidence showing damage from low lubricity(when proper filtration is used)?