I have a Negretta in front yard in ground ,here in 6B,and it did not die down last Winter tho no Winter protection was given.
It was like three foot tall,i sold a few cuttings out of it,and the remaining 20 inches was alive in Spring tho we had 0 F low extreme a couple of nights.
So ,my opinion is,that the tree is about as cold hardy as Mt Etna type tho is different and got more ornamental leaves and pink leaf stem,in my climate.