[FARWEST]

Yeah...no fuel savings in twin torbos and an intercooler upgrade...maybe a 15k outlay and a true 'beast' of a ride as an outcome.
I cant compare your fuel figures with my old oil burner...the majority of members that have the oil burner engine bought the bus as a tow vehicle...caravans behind diesel grands are a good match...fuel figures 'hooked up' and driving 'accordingly' equal (roughly) your high figures.....8's on open highway averaging 10's around town is commonplace...having said that...the diesels HATE having the majority of their drive life in urban mode...first hand knowledge and NOT uncommon or something that hasnt been discussed in depth here....even to the point of members having a 'town car' and the deisel Grand as a tow/holiday/distance bus.
Its back to researching just what ECU Upgrade/Chip/Tune will suit your needs...they are available and the costs have been disclosed.
The 'mystic' economy boxes are not recommended for genuine assistance.