Highest point of elevation in the Keys is 18 ft I think so good reason for evacuation. Storm is taking a similar path as the Hurricane of 1935 which wiped out the Keys and killed 600 people when a hurricane with winds of 200 mph hit.
There is just no where to go when a storm of magnitude hits down there. A 20 ft storm surge from Ivan could easily wipe out 90% of the property down there.