If someone is solidly in 7a, with occasional dips into 6b, I think they could grow Tam Kam successfully.
I’m theoretically on the border between 7a and 6b, but based on the last 5 years, I think I’m on the border between 6a and 6b. Mine survived the first few winters, which ranged from 0F to -4F (6b) without any die-back. Then -8F and -9F (6a) wacked it good, leaving it to grow back from the base, where I piled wood-chips. This past winter, I took some time and bundled it up well (tarp, leaves, etc), so of course we had another (relatively) mild winter around 0F.
I’ve got a bunch of suckers growing up around it, which I’ve grafted interesting varieties to. So I probably won’t bother to protect Tam Kam this winter. Either it lives and maybe I finally get to sample some (it bloomed for the first time, right before the 2 cold winters), or it dies and I let one of the suckers grow up and replace it. The suckers are mostly Americans or hybrids (Nikita’s Gift, Yates, Early Golden, H118, etc).