I was tempted to give a similar answer, but I'm not so sure it's irrelevant. Yes, the windchill won't affect the energy efficiency of the heatpump. But it will affect the rate at which the house loses heat, and thus the BTU's the system needs to provide to maintain a temperature. If the heatpump has been exactly sized to maintain a house temperature of (say) 70F at the expected winter low, adding a strong wind on the exterior of the house seems like it might make for a chilly morning. The effect isn't as strong as it is for bare skin at body temp, and hence the exact numbers will be off, but it's still there, right?