Watch the units over 15SEER, EER doesn't improve much over 14 regardless of SEER. SEER is rated at 82f, EER is rated @ 95f. Do ask for demand defrost for a heat pump, it will add real world efficiency by going into defrost only when needed. Look at the COP ratings for the heat side. Units are rated at 47 and 17 degrees. I wouldn't expect much output below 17.
Here are some data charts, I wish all manufacturers would provide tech data on their public website.
Ruud 15 SEER