Curious to see what exactly the foam framesheet was doing for me, I pulled it out. Previously I was folding my BA insulated Air Core in thirds widthwise (two chambers wide) and then in quarter lengthwise and sliding it into the hydration sleeve to supplement the back. Turns out that folded the same way, it fits into the framesheet sleeve perfectly. I took it on a morning hike loaded with about 24-25 pounds to see what it felt like compared to the break-in hikes I did with the framesheet installed.
Long story short: I couldn't really tell a difference. I'm sure it could get pretty uncomfortable if carelessly packed, or heavy enough to sag and buckle the folded mat, but I doubt it's any worse than other UL frameless packs, especially those that don't have the 3D mesh panel for padding. My frame sheet weighed 72g on my scale, add that to the long straps that got trimmed short and a few pieces removed, my Jam has lost 4.2 ounces from the day I brought it home.
I'm not convinced it'll stay like that, but it was a neat proof-of-concept for the summer trips where I'm cutting every ounce possible from an average 12 lb base wt.
I might think about trimming the factory one down and gluing a spar to it for heavy loads.