Well I live in Australia, with high heats, and sometimes high humidity. I have a trainer (Boomerang) that I have keep in my shed/workshop which gets very hot in Summer (too hot to work in). It has been in there for 3 years, and I have noticed recently that the covering has started to come away from the balsa, and it looks awful. Also, the rubber bands have all perished (they are hard and if you stretch them, they break). No good if they are keeping the wings on. The foam wheels aren't affected at all. The battery seems OK, but it is an older type (not LiPo, NiMH I think).

Unfortunately I can't tell you about the foam on your foamies, as I am not into these, but I can give you anecdotal evidence that the foam wheels are fine.