Hi joebk,
The layer of polyfoam will also have a lot to do with how soft the topper feels because with 2" of 4 lb memory foam you will “go through” it and feel the material underneath it. You may not have a choice of how “soft” the memory foam feels because memory foam is not generally sold in different firmness/softness levels even though different types of memory foam are softer or firmer than others. In general though … any 2" layer of memory foam would be fairly soft when it was warm.
Based on “averages” and not on your own personal testing or on your body type or sleeping style … I’d tend towards a memory foam with a medium/soft layer below it so that you have a more gradual transition between layers. Don’t forget though that you are looking at a combination that you haven’t personally tested and combinations of materials are less “predictable” in terms of how they will feel for a specific person than single layers so your own experience may be different from any “theory”. Both of these would be part of your comfort layer so the combination as well as the top part of your current mattress would need to provide you with the pressure relief you need and “fill in the gaps” of your spinal curves … especially in the lumbar area. If the comfort layers are too firm (as a combination) … they won’t provide the pressure relief you may want. If the topper layers in combination with the current soft layers in your mattress are too thick … you may be too far away from the support layers and could compromise alignment.
Both 2.5 lb and 3.0 lb polyfoam can be HR foam and would both be good quality. In choosing between these … the specs and “performance” of the foam would be more important to me than the density (which is good in both cases here). If the 2.5 lb foam is HR foam and it comes in the ILD you want … then it would be fine. If it is just a higher density of HD foam (it doesn’t use the specific combination of chemicals that produces HR foam) … then I would go with the 3.0 lb.
HR foam has a higher resilience and a higher compression modulus (sag factor) than HD foam and both of these make it a higher performance choice in a mattress outside of just durability issues. Some lower density foams can be “high performance” (similar to HR but lower density) and some higher density foams can be “conventional” foams (similar density to HR in density but not the same performance).
Some polyfoam sources (outside of any local foam shops that may be close to you) are in post #4 here.
Phoenix