Hi sleepsleep,
This is of course speculation but it is very possible. it would also depend on the design and layering of the memory foam mattresses.the reasons you mentioned is certainly one of the reasons that some people just don’t like memory foam mattresses … especially the ones that use slower recovery types of memory foam in thicker layers.
There is no way to really know this for sure without lying on the mattress in person because the type of material that is in a mattress is only part of the picture. If the layering of a mattress … regardless of the materials it uses … doesn’t give you the PPP (Pressure relief, Posture and alignment, and Personal preferences) that you need and prefer then it would still not be the “right” mattress for you. The type of material is more of a preference and regardless of what type of material is in the mattress the layering used (the thickness and softness/firmness of each layer and component) still needs to provide the basic functions of a mattress for your body type and sleeping style. This is why your own personal experience and testing will always be more accurate than any “theory”.
There is also no “secret” in the design of the Tempurpedic Weightless mattresses. Many other manufacturers have been making mattresses with a similar design for many years and Tempurpedic is a relative newcomer to this type of layering. One of their mattresses called the BellaFina did use latex under the memory foam until they stopped selling it but it used a thicker top layer of their firmer 5.3 lb memory foam over the latex. You can read a little more about it in this thread.
The odds are good that you wouldn’t have the same sinking sensation as a thicker comfort layer that only used memory foam but I would also bear in mind that different people can have very different perceptions of the same mattress and lying on it in person is really the only way to know how it would feel and perform for you.
Phoenix