Hi needanewmattress,
It depends on what you mean by support. The upper layers are primarily for pressure relief and for secondary support. Primary support which is what most people mean when they refer to support comes from the deeper layers of a mattress (I posted some links in my earlier replies about primary support, secondary support, and their relationship to pressure relief).
There is more information about the most important parts of the “value” of a mattress purchase in post #13 here. If a mattress is a good match for you in terms of PPP and you prefer it over a latex mattress or you are in a lower budget range where latex isn’t a possibility and there are no weak links in the mattress in terms of the quality of the materials then it would certainly make a suitable choice. Tuft & Needle and Dreamfoam bedding are both members here and both of them sell a polyfoam mattress that many of the forum members have purchased and done very well with (see post #2 here).
2.6 lb polyfoam such as Everflex is a very high quality and durable material so in terms of quality and durability there would be no weak links in a mattress that used it (see the foam quality guidelines here. There is also more information about the different types and grades polyfoam in this article about polyfoam comfort layers and this article about polyfoam support layers).
If a polyfoam material is made in North America and/or is CertiPUR certified for harmful substances and VOC’s then I personally wouldn’t have any concerns about toxicity, cancer, or other health issues no (polyfoam is the same material that is used in most furniture foam). For those that may have unusual sensitivities or health conditions such as MCS (multiple chemical sensitivities) and wish to avoid synthetic materials that most people would be fine with then there is more information in post #2 here that can help answer their questions concerning “how safe is safe enough for me?”
Phoenix