I don’t buy the scenario that using an additive elevated MPG by that much. If that were the case the refiners would be required by Federal law to add it to the gasoline at the refinery. Placebo.
As to Top Tier gasoline, I’ve never noticed any difference at all in MPG between Top Tier or the lesser brands and I monitor fuel mileage religiously; as a means of keeping tabs on any engine performance drop-off more than anything else.