Do you mean Cordyline australis? Im pretty sure thats the most cold tolerant Cordy and it is really more of a zone 8 plant. But the good news is that I have seen them survive winters here in a zone 7, but extremely wet, snowy, windy, or cold winters knock them out easily. They should be protected in your zone just incase but I dont think they will have a problem during the average winter by you. If you see people growing them as large plants, then they are probaby more than able to survive your winters. Also, the green form is MUCH more cold tolerant than the red one. I had a red one and it survived down to about 20F, but the green form survived down to about 10F-15F. There is a brownish-red form that is suppose to be cold tolerant (possibly more cold tolerant than the green form), but I dont have any personal experience with it.
Good luck!
-Alex