If it's making 55psi of boost and suffering from poor fuel mileage verify the exhaust isn't plugged. Also you need to monitor readings on CAT ET to make sure readings are within spec. In order to achieve boost you need fuel so it makes sense both of these symptoms happened simultaneously.
It was a combonation of a high fuel temps around 185-190° from low fuel capacity at only 125gals full after wet system/split tank install, a really ####ty tec that had the overhead all over the place and had deleted the injection calibration codes from the ecm, etc...had overhead and ecm straitened out and added a fan cooled fuel cooler in the return line. Dropped fuel temps 50°+- and helped big time.