Here's some info on mahoniasoftcaress.com
Maybe not too hardy.
During periods of winter frost, it will be necessary to move a mahonia shrub in a pot to a more protected place, preferably a frost-free location. Temperatures just below freezing, however, should not cause the plant any permanent damage. But if temperatures drop to -5 degrees Celsius, a frost-free location will definitely be required! In the garden, it is important to shelter ‘Soft Caress’ from easterly winds. During periods of -10 degrees Celsius, it will be important to cover the plant with a winter-protection fleece.
I think I read that a late spring frost can cause issues as well. Some say it's ok for down to -15c. I'll move it out of the cold winds tomorrow. Belt and braces.