A heatpump really starts losing the ability to deliver warm feeling air as temps get lower. It is still energy efficient and warm enough to heat the home, just does not feel warm with maybe 85F air blowing. That was the reason that the "luxury" system was installed 8 years ago. It is more comfortable and feels better using the fuel oil backup at the lower temps. Reducing the lock out point from 45F to 32F will really cover a great deal more of your heating needs with the much less costly heatpump source. Changing that lockout point should not be much of a challenge so that it can be experimented with. Good Luck.