Hi nomadmax,
Welcome to the Mattress Forum! :).
Hopefully some of the members here that own a Tuft & Needle will see your post and share their comments. It uses polyfoam in the comfort layers which would generally be cooler than memory foam but some of the other variables that can affect sleeping temperature that are discussed in post #2 here may also be helpful. The Tuft & Needle is a firmer mattress and the polyfoam is 1.8 - 2.0 lb density which is a good quality material so outside of the normal break in period for any mattress it will tend to maintain its firmness for most people (unless they are in much higher weight ranges) for longer than the mattress has been sold.
It sounds like you may have chosen a mattress that was on the edge of being too soft for you when it was new and that the normal break in period over the first few months where a mattress will soften slightly was enough to “put you over the edge” of the range of softness that is suitable for you to sleep on (see post #2 here). Unless you are in a much higher weight range and the Tempurpedic you had used thicker layers of 4 lb memory foam (in which case durability could become an issue) it’s very unlikely that the memory foam would have softened more than normal because the memory foams that Tempurpedic uses are good quaity and durable materials as well. The softness and thickness of the comfort layers will also contribute to how much you sink into the mattress and particularly with memory foam, thicker and softer comfort layers can also increase sleeping temperatures.
Phoenix