Hi alpharaplpha.
It sounds like you’re considering making your own mattress with 6" of 36IFd polyfoam as the top layer. 36IFD would certainly be a firm upper layer for a mattress, but based upon your own PPP it may be something you prefer. Here is a chart (section 4.2.2) from the Polyurethane Foam Association that relates different typical IFD values.
If you are attracted to the idea of designing and building your own DIY mattress out of separate components that are purchased from one or several different sources then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be useful as well.
There is more about the pros and cons of different types of innersprings in this article and in post #10 here.
Overall, it’s not possible to tell how a mattress will feel and perform for any specific person based on the specs or density of the materials and it would depend entirely on the person and which mattress design was a better match in terms of comfort, firmness, and PPP. For some people one would be better and for some the other would be better and for some neither one would be a suitable choice. The only way to know would be based on your own careful testing or your own personal experience when you sleep on it.
You are making a risky purchase with your Signature Sleep mattress because the foam layers on either side of the spring are a very low quality/density material (1.25 lb polyfoam) that could compromise the durability and useful life of your mattress/topper combination. A mattress like this doesn’t meet the durability guidelines I would suggest, even though these foam layers would be deeper in the DIY mattress you’re considering creating.
Phoenix