Hi DesertDuck,
I’m not sure where you read this but I certainly wouldn’t agree with it. Assuming that a memory foam layer meets the minimum density guidelines I would suggest for your weight range the specific properties of any memory foam formulation (regardless of density) would be much more important to me than any potential durability benefits of an additional .3 lb density (see post #7 here).
I would also keep in mind that a warranty only covers manufacturing defects in a material which generally involves visible impressions that are deeper than the warranty exclusion (which would be uncommon) and not the gradual (or more rapid in the case of lower density materials) loss of comfort and support over time that comes from foam softening that will be the most likely reason you will need to replace a topper. A warranty will have little to do with the durability or useful life of a topper.
IMO it would be unrealistic to expect any topper to maintain its comfort and support for longer than the 10 years that your Sensus topper lasted you (and less than that would be more realistic for a topper which will tend to soften and break down faster than the same type and quality layer inside a mattress cover) and I would consider anything more than that to be “bonus time” rather than an “expectation”.
I do think highly of foamorder in general but they would be the most reliable source of guidance about how the properties of their specific toppers compare to other memory foam formulations on the market (such as Tempurpedic memory foam) in terms of temperature sensitivity, response rate, and firmness.
Everflex is the name they use for their polyfoam not their memory foam.
Phoenix