I overwintered a pomegranate (Wonderful) in a 7-gallon terra cotta pot here in zone 7B with no noticeable dieback. It spent the winter under a small greenhouse frame covered with a heavyweight frost blanket with the rest of my potted fruits, and I mounded leaves around the pots to help add a bit of insulation. My guess is that the temp never dropped below the mid 20s under the cover. Anything lower than that and I'd be concerned. If you haven't already purchased your plants, you might consider going with one of the hardier Russian cultivars.