There is a heat pump technology called 'enhanced vapour injection' which appears to alter the capacity (and perhaps efficiency) curve across various outdoor air temperatures.
See page 3 in this doc:
http://www.emersonclimate.com/europe...PRESSORS_0.pdf
Or see figure 5 in this doc:
http://docs.lib.purdue.edu/cgi/viewc...&context=iracc It shows better COP at lower temps. Heat pumps with such technology would perform -a little- better than that spreadsheet's adjusted HSPF values would indicate, in colder regions. Figure 4a confused me for a moment!