As I posted before - I think they are mostly warm weather creatures.
However, about 4 years ago a young stray adopted us, and he grew to be more like a Maine Coon, and he definitely suffers in the heat. We've had to give him a lion cut the last three summers, but because he needs to be under anesthesia and our funds are very low we are finding cooler places in the house for him.
I just now brought him an ice pack.
So, I guess it really depends on the breed of cat and how much fur they're wearing. :)