Hi toddking,
These types of questions are usually best asked of the source which is the manufacturer themselves but I think that post #2 here and post #6 here should be helpful.
In their Brooklyn Bedding mattresses (and I would also confirm this with them) … the lower of the three models in their exchangeable layer lineup also uses 1.5 lb base foam and the top two models use a higher quality 2.17 lb Omalon polyfoam.
Density is the primary factor in durability but there are also other factors which have a lesser effect which includes the firmness of the foam, the cell structure, the chemicals or ingredients used in manufacturing, any convoluting or surface modifications, and the foam above and below a layer. Omalon is a polyurethane foam that comes in a range of densities and is higher performance than conventional foams. While density is still a primary durability factor … the other durability factors in the Omalon would be superior and it is also a higher quality and performance foam (outside of just durability) even though it has a lower density.
Perhaps more important than all of this is that the support layers are not normally the weak link of a mattress anyway because they are subject to less stress than the upper layers which are the most important in terms of the durability of the mattress.
Phoenix