I just finished up my IX underquilt today thanks to this thread.
I tested it out in my basement and I think I'm going to add the draft collars to the head and foot ends to help with the sealing. I followed the instructions and dimensions posted at the start of this thread. I"ll try to post some pics after I finished working this week.
One thing I did that simplified the sewing process was to first pin and sew just the three IX layers. After that then I pinned on the top and bottom ripstop layers and sewed those on. Trimmed things up a bit and then sewed on the grosgrain last.
My IX underquilt came in at 17 oz including shockcord and cordlocks. I folded it longways twice, then rolled it up and it looks like it packs down to about 6" dia x 12" long. In a sack it probably would pack down a little better than that.
I think it's going to be perfect for 3 seasons when my 20F down UQ is just too much.