Hi Sleepy103114,
A mattress or “sleeping system” will generally tend to soften and break down from the top layers down because ongoing foam compression and deflection is the primary reason that foam materials will tend to soften and break down over time. In an apples to apples comparison between the same types of foam … softer foam layers will compress more than firmer foam layers and layers that are closer to the top of the mattress will tend to compress more than layers that are deeper in the mattress so any softer foam layers or layers that are closer to the top of your mattress will generally be less durable than the same type of foam in firmer versions or that are deeper in the mattress. In other words it would probably soften to “some” degree over time but how much it would soften and how much any softening would affect you in terms of PPP (Posture and alignment, Pressure relief, and your own Personal preferences) will depend on how much the polyfoam core is compressing under your weight “through” the layers that you have on top of it and where you are inside the range between “I can sleep on anything” and “princess and the pea”.
The thickness of a mattress or any individual layers inside it is really just a side effect of the design and the design goals of a mattress and is also only one of many variables that can affect the feel and performance of a mattress relative to any particular person and by itself isn’t particularly meaningful (see post #2 here). In some cases higher weight ranges will sometimes do better with a mattress that is thicker than lower weight ranges (see post #14 here for more about the effect of thickness) but even this depends more on the specific design and combination of materials in the mattress and on how well your testing or personal experience indicates the mattress “as a whole” matches your specific needs and preferences in terms of PPP than anything else.
2.6 lb polyfoam is a good quality and durable material in any thickness but the “bottom line” is that the only way to know whether adding another layer to your sleeping system will be beneficial, detrimental, or neutral or will be a better “match” for you in terms of PPP will be based on your own personal experience when you sleep on the mattress because there are too many unknowns, variables, and personal preferences involved to be able to predict this based on specs (either yours or a mattress) or “theory at a distance”.
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 helpful as well.
If you are sleeping well on your mattress now then I personally wouldn’t argue with success and would hesitate to make any additional changes unless you have a specific reason to do so. If you are adding additional foam layers to your mattress then you would also need to use a thicker mattress cover as well so if you have already purchased a mattress cover you would need to replace it with one that is thicker.
Phoenix