Why would you think that's least efficient?My manual J had my heat loss at design conditions to be 34,000. So it is true I could get by with only low stage, but why use the furnace at its least efficient?
What is your static pressure at low, and high?
What is your duct leakage at low? At high?
What pressure imbalances occur in your rooms at low? At high?
How much pressure induced leakage is occurring at low fire? At high?
How much heat gets delivered to occupied space per therm burned on low? How much on high?
And at what temperature is comfort achieve with a low fan rate? How much warmer must the setting be for a rapidly cycling high blowing fan rate?
You aren't buying temperature, you are buying comfort. http://bit.ly/comfortcalculator
If you look at this just looking at one tiny slice, it's like your nose is pressed up against a work of art. Would you really do THAT and think you were seeing the whole picture?