Hi Cloud999
Thanks for measuring the thickness of the free 1.5" latex topper you received. Good to know! 
This certainly adds a bit more depth and contours better around your body and helps with the pressure relief that you need. I am not sure now if Zen changes toper specs from time to time as other members of our forum have mentioned receiving different topper thicknesses. Good to keep an eye on!
You are quite right that the inconvenience and the time involved in researching and “switching” mattresses (which may or may not be better than the previous one) should always be “questioned” as part of your personal value equation and what is most important to you. Most of the people that come to our forum wrestled with a similar situation …. the choice between something that seems good enough and something that may be better. I wish there was a “formula” that can be used to assess it, but unfortunately, no one else but you can answer this with certainty as there are too many variables, needs, and personal preferences involved.
[quote]I’m a little concerned that for my BMI (5’11", 200 lbs., 27.9), this mattress is a bit soft in the lower support layer (N3, 25-29 ILD).
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With a two-sided design, the softer layers located lower/deeper within the mattress work together with the layers above for a certain feel and support/comfort range. If durability is of concern, I’d keep in mind that these “deeper” layers compress uniformly and are not under the same mechanical stress that the uppermost layers go through and they would be the last layers in a mattress to break down over time. Additionally, the durability of the latex in the comfort layers would not be of concern at your current BMI. The only concerns I’d have would be only for a plush latex comfort layer for a person with a 30+ BMI index which would be the threshold where you would start to see some evidence of breakdown. If suitability is of concern to you then I’d keep in mind that the softer layers situated much deeper within the mattress (or at the bottom) are the least noticeable on the sleeping surface and the foams beneath the N4 will only very slightly soften up the deep support, If you found the layering arrangement is a good “match” for you in terms of comfort, firmness, and PPP you should be OK at your BMI and there should be no cause for concern for either support or durability.
Other than this It is not possible to tell if this layering combination is a perfect match for you as you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to assess this, predict, or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here ).
I hope this clarifies things and helps with your final decision
Phoenix