Short answer-There are two ways you could mod the backpanel: the easy way, and the OCD way. I chose the latter.
Simple way is just unstitching everything without touching a single pack seam, whereas the OCD way is partially undoing some of the seams to remove as much fabric as possible, and to get a cleaner overall appearance.
Simple way: Get in there with a seam ripper. It'll take you maybe 25-30 mins tops. Just take the seam ripper and remove all the stitching that holds the black binding tape on the borders of the padding. Once you do this, you can easily remove the foam padding material underneath the wicking mesh of the padding. For the lumbar pad, seam rip as much as you can, and once you get to the area where the lumbar pad meets the pack bottom's seam, just trim it with scissors, so you don't have to actually undo that bottom seam. Same thing with the shoulder strap padding...seam rip all the black binding tape on the padding borders, remove the foam padding, and get close to where the shoulder straps are sewn to the pack, and trim with scissors there, leaving a bit of raw fabric exposed.
OCD way: I wanted to remove as much weight as I could, so I didn't want any of the padding material left over. That meant I actually undid the pack bottom seam to fully remove the lumbar pad. I also didn't want to leave any raw edges, so I used some of the binding tape material on the shoulder strap padding to encase the raw edge where I trimmed the padding. You can see that in the pics...it gives it more of a finished look. I had to sew on that binding tape material across the trimmed edge of the shoulder strap padding, so that meant I had to undo the side seams enough so that my sewing machine could get the right angle.