I "think" Marty may have also suggested running some D2 for a couple of tanks and doing a comparison whcih I would agree with.
Mine gets 31 mpg burning D2 and 27-28 on B99. Burning B99, I hear my turbo a LOT more than I do when running D2. In my mind this means I am burning more fuel to achieve the same driving style I have when running D2. My commute (110 mi/day) is pretty consistent day to day and works out to 20/80% city vs highway. My feeling is that if I burned an entire tank of B99 in city driving conditions that I might get as low as 20 mpg on B99 due to a heavier foot.
You may be more enthralled with your turbo and kickdown switch than I and therefore are achieving even less mpg than I???
