Hi jodebg,
If you are considering a DIY design then I would make sure you’ve read post #15 here so that you have realistic expectations of success and the trial and error process and costs that may be involved.
That would depend entirely on the specifics of the polyfoam in each mattress (density, IFD etc) because the thickness of a layer is only one of many factors that can affect the feel or performance of a material. Your own personal testing or experience in terms of PPP is really the only way to know whether one would be a better match for you than the other in a particular mattress design. In very general terms if two materials are exactly the same and all the other layers and components of a mattress are also the same then a thicker support core would generally act a little softer and have a greater range of compression between soft and firm (see post #14 here about the effect of thickness) but it’s open to question whether you would feel a difference of only 1".
[quote]To compensate for the missing !" layer of soft foam sewn into the bamboo cover that is not available, is adding a 1" layer of
soft poly to each side of the mattress a good idea? Any disadvantages?[/quote]
Again … this would depend entirely on which one you preferred in your own personal testing. The types of changes you are considering can have some very unpredictable results because foam that is used for quilting will feel different than foam that is used in a comfort layer and changing the thickness of the layers in a two sided mattress will change both the support and comfort of the mattress. Outside of the quality of the materials … the advantages or disadvantages would depend entirely on which one your personal experience indicated was a better match for you in terms of PPP.
If it was me and there was a zip cover I would leave the layers loose so you can adjust them and experiment with different combinations or exchange layers based on some trial and error (see post #15 here). You can always glue them after you have found the combination of materials that works best for you based on your actual sleeping experience.
Phoenix