I finally got the chance to test out the "Feed Sack Pack" on a real hike and it worked pretty darn good too. I did about 12 miles through some pretty rough terrain, not quite bushwhacking, but pretty darn close to it. I had it loaded with about 30lbs with bramble and branches trying to snag me most the entire way.
I didn't baby the pack either. I leaned up against trees with it along the way to help relieve the load while I waited to catch my breath, set it on the rocks and ground at other times, and generally treated it like I did when I had the factory bag attached to the frame.
The "Telescoping" design using a "bottom" sack inside a "top" sack worked out great. As I shed clothes I just lifted the top sack a bit to make more room to fit them inside. The compression straps attached to the frame kept the pack tight to the frame and all the gear snugly inside it. , And I attached a couple of "CargoLoc Tarp/Canopy" stretchy cords between the compression straps to hold my butt pad and trash bag on the outside of the pack.
I also bought a cool little drybag for my GPS at Wal-Mart in the camping section. It already had a belt loop on it that fit my hip belt straps perfectly, and I had a water bottle holder already sitting on my gear shelf for the other side.
I ended up making a sort of "Fanny Pack" out of the feed sack material that I clipped on the lower part of the pack to carry my water, fuel, and some snacks for easy access. It uses the same dry bag closure and worked out pretty good too.
So far it's not showing any wear after the hike so I'm pretty sure it will last long enough to make it through the season, but burros will make sure I'll have some spare sacks soon to replace it if it doesn't.
Maybe most important, I think it was quite a bit easier to carry. I could adjust the load better, and keep it tighter to the frame. The pack didn't shift around at all and it only took a bit of adjusting to get it nicely balanced.
Since I could get all my gear inside it, and placed what I wanted to on the outside, and it was easier to carry, I can say that for me it's a pretty nice upgrade. I don't think I'll be putting the old sack on that frame again anytime soon.
At the very least been a fun project, and it inspires me to keep poking around with making my own gear.