I think it important to consider that a healthy animal likely could handle cooler temps and bounce back more quickly, but don't have any proof to back this up. Also, I would say that they could likely handle colder than what you have stated. I remember a few years back seeing the iguanas literally frozen dropping out of the trees in the south of Florida and the next day they simply warmed up enough and walked away. Even last night it was down at or below freezing and the little lizards in the yard are starting to scamper around again. I'm sure most found slightly insulated hiding spots for the night, but it was definitely cold.