Actually, if I have the heat on for hours the seat gets a bit uncomfortable but I think it is because my arse is starting to burn up more so than the structure of the seat itself. However, you do raise some interesting points since it makes sense that the characteristics of the closed cell foam would be altered by heat. When it starts to get uncomfortable, but it is still cold out, I switch to the low heat setting and that solves the problem.
With the heat off I have not noticed any greater comfort after 30 minutes or so of riding.