its been my understanding that the tuner improves power and economy in every tune...and you can even modify your stock tune. everyone ive talked to with a tuner reports better mileage in pretty much every tune setting.
for what its worth, i was running 93 octane in my truck before and after tuning it...so i didnt go from stock with 89 to modded with 93...that increase is tuned and untuned, with 93 octane fuel.
one other example...those of you who play with your overhead readout likely know where their trucks "Sweet spot" is...i find that i get my best mileage with the cruise set at 60-65mph. at that speed on a relatively flat highway, my computer readout stays around 22-23mpg. at those same speeds with the tuner, my mpg hovers around 25.