Not sure if I'm reading the question right, but the fit feels different because of the different suspension system. The T1 feels like it moves more with you keeping in mind the two internal stays, while the Duplex feels more rigid with the external type frame.
From my research you can get a little more barreling with an overstuffed T1. With the DT1 you can stuff it as full as you want. The only way to barrel the Duplex frame is by major over-tightening of the bottom compression strap. So much that it actually bends the frame sheet around the bag. Does that make sense? Either way I doubt a little barreling would decrease the comfort of a Kifaru because nothing will touch your spine. The torso pads might hit your back a little differently but that's all. Maybe somebody with more hands on experience can back me up or prove me wrong.