If you use WEST epoxy or similar, this is definitely a problem. There are other epoxies which will hold up to a higher temperature, particularly after post curing. I think that the typical glass transition temperature for this general sort of epoxy is around 140 or 150 F. Probably worth finding out exactly what epoxy was used.
Extruded styrofoam might have some temperature issues too. Isn't that what the Varieze uses? Just looked at a spec for one of the Dow extruded styrofoam products, and it lists a maximum use temperature of 165F.
I suppose if you live in Denmark or something and never fly south, you could be ok.
Interesting data. Your OAT was pretty low. I imagine on a 100 degree day, there could be a real problem on the gray surfaces. Definitely shows us why people use white paint, particularly in Arizona.
Some of your nighttime measurements seem to indicate the error is small, but when I was helping a guy study temperatures in electronics with an IR thermometer, he said you really needed to calibrate for the properties of the particular sort of surface you were dealing with, as they don't all radiate the same. I'll admit I don't know a lot about it.
So the issue with white paint vs. heat isn't so much because of the weakening of the epoxy, but the weakening of the foam? That would make sense, as I've looked at the thermal limits of some epoxy; it's at like 200+ degrees F if I remember correctly (I thought it was West Systems actually, apparently not).
I have a background in architecture and construction management. In high rise construction the rate of thermal expansion of aluminum curtainwall components is critical. From memory, painted alum. will heat up 140- 175 degrees depending on the darkeness of the color.
As others have noted the temp. that some epoxies soften and deform (TG temp) is often as low as 120 degrees. I build vacuum infused canoes and ruined a female mold by heating it up to 145 degrees F. to aide in cure. A Lesson learned the hard way for sure.