I posted this question in the Maintenance forum a while back, and the general feeling is light modifications do nothing to your fuel economy. Look at stan (satanic) among others, they're getting around the same (13L) and they have alot more mods and power than my stockie. I guess if you want to control boost via your foot, you end up with roughly the same mileage out of a tank.
Having said that, on the stockie.. even with giving it some on cruises, i don't notice any increase in fuel usage. That might change when I wind up the boost eventually though.