Since nobody else has chimed in with firsthand knowledge, I'll add what I know...All D. binata forms (except possibly multifida) are supposed to be able to survive hard freezes. The temperate forms will form hibernaculae, while the tropical forms will die back to the roots when exposed to freezing temperatures. I've read they can survive down to 15F, but I'd be a bit wary about testing that, especially in a smaller pot.
I have no knowledge of the winter hardy form from ISCPS, however all the binata forms I have, get frozen every year in our UK winters in outside bogs. They generally come back from the roots fairly readily, as do the Capensis, but obviously mature later in the year. They have had temperatures down to -15C for short periods.
I have no absolute knowledge of a temperature that they can survive, as your question suggests there are different clones out there that have varying degrees of tolerance.