I know that the Ram Cummins 68RFE does not give full power or lock up the torque converter until 4th gear when accelerating. A locked torque converter will send 100% of the engines horsepower to the transmission, and an unlocked torque converter will not. This is one major reasons why a Cummins feels so sluggish off the line versus a Duramax.
My custom tune locks the TC from 2-6th and makes a world of a difference in the trucks off the line acceleration without adding much power to the engine. Once the torque converter locks, the truck just pulls like a train. Without it locked, it just makes a lot of noise at higher rpms going nowhere fast.