When I was trying the ultralite thing, I found a way to justify my 6 ounce chair kit. I would leave the thermarest Prolite pad (not the Neoair) in the chair, let the air out, fold it in half, and roll the edges inward until it was the same width as the back of the pack. Insert it down the back of the pack and, voila!, you've got stays to transfer the weight AND a comfortable place to sit in camp. If that wasn't stiff enough, I'd blow a bit of air into the pad to stiffen it. With the 3/4 pad, there was always about 4 inches that had to be folded over underneath your legs; this meant that the folded pad was also a bit thicker at one end. That end went at the bottom of the pack, and made a passable lumbar pad.
I used to do this with the Granite Gear Virga, and it worked pretty well up to 20 pounds, and at 21 pounds was completely insufficient. That had mostly to do with the fact that it had a very narrow webbing hipbelt that wouldn't support more weight than that without crumpling.
I always thought a lightly padded, wider hipbelt and a hydration sleeve inside the pack would have made it perfect: the pad could slip down inside the sleeve, and the hipbelt would put the weight limit toward 30 pounds. I see they've now added the hipbelt.