Jamie's elegant math got me curious. I would have assumed the pad took up more space, so I tried it. I don't own a full length pad, so I put in 1/2 of a ridge rest and half of a 59" GG pad. Note that this combo is a little shorter than a true full length pad. Of the original 9.7 inch diameter, there was about a 5 inch diameter tube left in the middle once the pad was in place. The pad reached to the top of the pack, just below the extension collar.
I confirmed that I could stuff my summerlite sleeping bag, nano puff jacket, and 4 oz poncho tarp into this tube. It involved significantly more compression of the down than I generally like and these 3 items came to the top of the pad.Misc items bag could go in the back pocket, water on the side. Trapper's mug and food could fill the extension collar and would accommodate maybe 2-3 days.
The pack was no longer the "fluffy cloud" I'm used to, and I don't think I would personally enjoy using it this way for my 4 lb gear list(I rarely ever use the extension collar or back pocket)but it could be done.