I sat in both the leather and the Nuluxe and found them indistinguishable. Every person that has sat in my car so far as ASSUMED it is leather.
Now that doesn't exactly answer your question, but I haven't really had a blindingly hot day yet in Boston, so I'm not sure if it will act any differently than leather would.
I guess the one thing I'll say is that there are more light-colored Nuluxe options, so on a hot day they probably fair better than dark leather does.