I haven't *noticed* any problems with heat, but I can't remember whether I've really pushed it in hotter weather. Where I live now it rarely gets past around 29C. It gets warmer nearby -- the hills, mountains, and coastline make for a lot of microclimates, and I've been out on days that hit 40C, but I just don't remember whether I've used it outdoors when I've been in that kind of heat.
Now I'm curious, though. I think I'll make a point of bringing it with me more often, at least on hot days.