I think the issue is more accurately, lower gas mileage with regular. But there is more to consider than simply fuel mileage. Running regular in a car designed for premium is possible because the engine computer adjusts, among other things, timing, to avoid detonation. I learned my lesson in a previous LS400 that running regular over time causes the engine to run hotter and degrades various engine components. I believe that this was the cause of numerous repairs over the years.
Tom