Question: If you are happy with your Jam's pack weight when going on longer trips — wouldn't you be happy with the same Jam pack that's carrying a lighter load on shorter trips — if it could be made to carry that load in a more stable manner?
If a smaller ULA pack can handle your 2-3 days' load superbly, but happens to weigh about the same as your modified Jam — why are you suddenly dissatisfied? If the lighter weight is already delightfully light enough for you — then maybe the ULA's pack features (eg hip belt pockets) and durability (same beloved Dyneema as your Jam) are excellent trade off's for the weight??? Just food for thought, of course, and not saying that a ULA pack is the only way to go.
Another food for thought. The new Jam 2 has a supposedly very simple yet very effective way of shrinking itself significantly — by way of a small cord near the bottom secton of the pack. Before shelling out money on a new pack, why not go to your nearest REI, give it a look, and see if you can mimic this nifty feature in your own modified Jam? This might just be the simplest and most effective option yet!?! Again, just thinking out loud on my part…
Where are all the actual Mariposa users anyway???