An additional leaf will surely increase the spring-rate, carrying your rated load - more confidently. But adding a leaf will absolutely compromise unladen and lightly loaded ride quality - as well as any lightly-loaded off-roading articulation.
I had Timbrens for years, and took them off for cradled bags. The Timbrens offer some unencumbered flexibility. Maybe 1.0-1.5”. But then - ZERO FLEX. Attached-inflated-bags offer just a hint on unloaded flex (compression), then realistically limit out. (Some) Unattached-cradled bags can be entirely deflated, and driven uninflated, without any damage. Giving you realistically full factory articulation, or the option of increasing ride comfort up to your rated max payload, by running them inflated. In fact, if you’re good at math/engineering, and you run a modest rear lift block, you can get both (simultaneously).