Hi yatyat,
There is more about the effects of layer thickness (or mattress thickness) in post #14 here.
I could also link you to dozens of posts that go into much more detail about these and other parts of mattress theory and design and some of the specs involved but it seems to me that you are already on the edge of being overwhelmed and I believe that there are too many unknowns, variables, and personal preferences involved for the vast majority of people to use specs or “theory” to choose a mattress and perhaps the most common outcome for those who try is “paralysis by analysis” and “information overload”. Too much information and too little information can both lead to equally poor choices.
The only ways I know for 99% of people to choose a mattress that is best for you in terms of PPP is either with your own personal testing or experience or with more detailed conversations with a knowledgeable and experienced online manufacturer or retailer who has your best interests at heart and who already knows what you would otherwise need to learn through a great deal of complex research and enough trial and error to “translate” what you are learning into something that is meaningful for you in “real life” (see mattress firmness/comfort levels in post #2 here).
Phoenix