I asked this question a few times and I was always assured it wasn't a problem but not really convinced.
Another thing to chew on is the 40c rating on circuit breakers. In a sunshine state garage (where panels seem to be located these days) 40c is not that far from ambient air and inside the panel it could get quite a bit higher.
That will change the trip curve.
I also agree the electronics in these breakers will not like the heat.
It would be interesting to put a temperature probe inside a panel, button it up and compare it to ambient air a few feet away. One of those little indoor/outdoor thermometers would work well for this. Maybe try the probe in different spots in the panel.
I use one of these thermometers when I am designing computer cases. They are pretty handy to see what is going on inside a closed box. Some of the nicer ones will also log high temp.
