A Banks upgarde is unlikely to improve the fuel mileage of any late-model engine, which have already been highly tuned for economy and efficiency. The Banks packages used to improve mileage as well as performance, but that was primarily on engines without advanced electronic controls. With modern engines, the Banks packages are used primarily to squeeze out a bit more performance.
Frankly I doubt there is much you can do to improve mpg on your 400 HP Cummins, but you should check out Banks own reports on their Cummins improvement package. They do mention a small gain in mpg in their test vehicle, but caution that some drivers will not see it. See Banks-Cummins 8.3
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL