Hi Supravista,
Answering this would require a book on mattress theory and design because there are too many variables and “it depends” in any answer. If you are considering designing your own mattress vs buying a design that already has a track record and is already “proven” in the market or that you can test in person then I would first make sure that you’ve read post #15 here and the posts it links to so that your expectations of success are realistic and that you are comfortable with some trial and error and any costs that can be involved with the mistakes you may make along the way.
Your own personal testing or experience is the only way to know with any certainty whether a specific mattress design and combination of materials will be suitable for you. Outside of this your best chance of success with designs you haven’t tested are more detailed conversations with an online retailer or manufacturer that knows more about the combinations of materials they use in their specific designs than anyone else and/or has a good return policy if the choice you make is not as suitable for your needs and preferences as you hoped for. Post #2 here has more information and some links to some of the sections of the site that may be useful but these are only generic and not specific to any person.
I personally don’t trust the accuracy of the specs they provide. You can read more about Foam By Mail (FBM or the other names they use) in this thread along with post #2 here and post #2 here.
In general … 2.8 lb polyfoam would be a good quality and durable material but of course the density or durability of a material has little to do with whether it would make a good choice in terms of PPP because durability and density have little to do with softness or firmness or how a material “feels”.
Phoenix