That is probably about the limit for 100% but there are some units like Bryant/Carrier variable speed units that can operate down -15 or so before you have to rely solely on electric backup.
-15 to -18 is where the best heatpumps become ineffective at drawing heat out of the air. The best units out their can probably maintain temp in a house down to around 15 without electric backup. That said it depends heavily on set temperature, insulation, and how much air leakage you have. Also...