Hi rest9rms,
There can certainly be a noticeable difference between different types and blends of latex (such as synthetic vs natural and Talalay vs Dunlop) which can all have different properties and a different “feel” … even if they have the same ILD or thickness. There is more about the different types and blends of latex in post #6 here and there is more about some of the general differences between Talalay and Dunlop in post #7 here.
Every layer and component of a mattress (including the cover and any quilting material) will have an effect on every other layer above and below it and on the mattress “as a whole” so depending on the type and firmness of each layer you may find that you would do well with some latex mattress designs that have a 6" + 2" construction and yet other mattresses that also have a 6" + 2" design may be completely unsuitable for you to sleep on. The same would be true for mattresses that have a 6" + 3" design or a 3 x 3" design. It would all depend on how the specific layers interacted together with your particular body type, sleeping style, and personal preferences and sensitivities.
The only way to know for certain whether any specific layering combination will be a good “match” for you in terms of “comfort”, firmness, and PPP will be based on your own careful testing or your own personal experience when you sleep on it.
What you “rule in” or “rule out” would depend entirely on your own criteria and the risks you are willing to take.
If you are attracted to the idea of designing and building your own DIY mattress out of separate components and a separate cover 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.
For those who decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of PPP as a reference point or blueprint and try and “match” every layer and component in your reference mattress as closely as possible or use a “bottom up” approach (see post #2 here).
For me … the “risk” involved of choosing a combination of layers that is a good “match” would be much higher than the relatively small risk of receiving a defective layer which most suppliers would replace if it was obviously defective (rather than just the wrong firmness choice).
Phoenix