I guess you now have the indoor fan running. Great
you cannot get enough heat from your unit when its -1 outside at least not as much as you can when its 5.5
Have you checked the tempature difference across the indoor coil (with system on and fan running inside and out) at different outdoor ambents?
When it was 15 degrees inside what was the temapure difference of output air temp and the 15 degree indoor temp.