This is a very helpful thread for me. I am a volunteer in mountain search and rescue. For me, at age 65, being light is important. My days of 45+ lb. packs are over!
The Zpacks solution is very appealing to me. Right now, I have a super minimum hasty pack solution that uses just a Ribz front pack with the very basic SAR essentials that include GPS, radio, extra batteries, compass, signal mirror, whistle, GoLite poncho tarp, small FAK, trail marking tape, and other items. For water, I use small nylon water holders on my belt. I have a next level that also uses the Ribz, but adds a fanny pack with holders for 2x 32 oz. water bottles. This option allows me to add more essentials. (By the way, I really like the Ribz front pack....)
But, the Zpacks looks like it could work for me as an intermediate solution --- between the Ribz+fanny and my ULA CDT. I consider the ULA CDT to be useful up to 25 lbs. After that I have an older Gregory internal frame pack that can handle up to about 45 lbs.
I thank all of you that have posted here with these most intelligent and useful comments. I may just have to spring for a Zpacks Zero.