TheUF paper on it says hardy to 9b, but it was written in 1999. That 9b is now 10a, and 9a is now 9b. Here in 9a, there are duranta growing in protected places against south walls on the UF campus that survive every winter. They get killed back a bit, but they even survived the freeze of a couple of years ago, so I'd say if you cut it back and protect it well during cold spells, it would likely survive.

I have an volunteer avocado growing on the inside of my south courtyard wall that is 3 years old, and has survived everything the cold has thrown at it with no protection.

When I had to leave my large crown of thorns outside during freezes, I'd crumple newspaper and cram it down around the base of the pot as high up as I could get it, cover the whole pot with blankets and tie them around the bottom of the pot. They're much more cold sensitive than duranta. One year, my yellow one was killed back about halfway, despite my protection, but the one next to the house was fine with little damage at all with just a blanket thrown over it. The yellow one actually came back better than ever from the base. That was the year in 9b/10a that it went down to 25 for 5 nights and killed all my bananas and a bunch of other stuff.

During that freeze, I had a neighbor with a hedge of duranta, and they were burned back, but she cut them way back, and they came back out in the spring and grew like mad.

In my experience, aside from true tropicals, most plants are more cold hardy than the zone hardiness designations say. They sell duranta in garden centers here in Gainesville, not that that is a recommendation, but most of what they sell is at least as cold hardy as tropical hibiscus, and mine have survived for about 10 years here, from all accounts. They were planted by the previous tenant when she moved in and she was here 6 yrs. and I've been here 4.