In a solid zone 4 that gets to -34 F some winters (sorry I don't have the conversion to Celcius offhand) in north-central Maine at the U at Orano in the Littlefield garden, they found no damage, not even tip die-back on various winterberries (Ilex verticillata.) There is a PDF file available giving their reults of several years of hardiness trials that you might find helpful to download, and I know that the arboretum at the U of Minnesota also does hardiness trials, though I haven't found any downloadable info from them. I can't remember if the Littlefield Garden trials had Winter Red per se, and I know that in borderline plants, having northern rather than southern ancestry may help with hardiness. Winter Red has done fine for me, but my lowest temperatures are -22.

I checked and they do grow 'Winte Red" winterberry in northern ME, so it's probably worth trying, especially if you have a sheltered area.