I have the original Timberline internal frame, and a new Duplex Lite. The lite does feel a bit more rigid due to the frame sheet. The lumbar pad also feels more substantial. I wasn't sure I liked it at first compared to the T1. But, after packing it and adjusting the suspension a bit, I've decided I prefer it (except the weight gain). What I've found is that the comfort difference is non-existent up to 40 pounds. Beyond that, the heavier you go, the more the Lite shines. The stiffness is not restrictive whatsoever, and it feels more connected to your body. Some guys act like they couldn't tie their shoes or turn sideways, let alone draw their bow, with some packs. It's kind of silly.
The Duplex lite is a great blend of comfort, durability, and versatility (and ventilation). As a sidenote, changing the torso length on the Lite takes a FRACTION of the time that the old T1 did. It also "stands up" better on its own, due to the frame sheet structure. Love it!