Retuning diesel motor for more torque and hp

I got a diablo tuner for my 5.9 cummins dually and wanted the tuner for better fuel milage when the computer was retuned for economy I gained a extra 50 horsepower I asked the techs how could I get better milage by increasing horsepower, the reply was because our tuner tunes the engine better so better economy and a plus of more H P . now days the people who make a living pulling goose necks want the trucks E G R and D E F deleted cools engine better and provides cleaner oil but it defeats emissions