Hi aemarcha,
It’s certainly makes sense “in theory” and it’s very possible that adding a softer layer would improve pressure relief and secondary support (which is the type of support that “fills in” the gaps in your sleeping profile) but of course the only way to know for certain will be based on your actual sleeping experience. If your mattress is too firm to provide good pressure relief or secondary support then adding a topper could also be well worth considering.
I certainly wouldn’t focus on whether a mattress uses a “differential” or a “progressive” construction because these are only very general and broad descriptive terms for how a mattress is layered and there is no inherent reason that either one would be better or worse in terms of PPP and either one could certainly make a good choice depending on the specifics of the materials and components and the overall mattress design and layering.
If by “differential” you meant “zoned” then zoning can help with more challenging body types or circumstances but it will always depend on whether the specific zoning pattern is a good “match” for your body type and sleeping style. There is more information about zoning in post #11 here and the posts it links to.
I would always keep in mind that the only way to know for certain whether any mattress will be a good “match” for you in terms of PPP with any certainty will be based on your own personal experience.
Phoenix