Where's that PITA jerk TEDKIDD when he might actually provide some VALUE ya mean?
Unfortunately, I don't see anything but to support/reinforce the good stuff already stated:
Supply improvement / loss reduction - what is the bigger opportunity? Be nice to test the room for leakage to know just how big a part of the problem that is. Funny how people focus on more heat when sometimes less loss is the easier fix. Caulk the window trim, jambs, baseboard...Think I would spend my time and money on insulating and draft-sealing the room.
Can you describe your furnace and house size? Does your furnace have the ability to run near continuous, or does it fire and shut off? How far is the new room from the furnace? Can we see pictures of the duct?
Shortcircuit has a crazy sunroom on stilts. Because his equipment modulates between 60k and 12k, it basically runs if it sees a moth fart. Apparently that has made a huge difference in comfort and evenness, summer and winter. I thought that room would require a separate heat pump, looks like I was wrong.
He's also got energy bills 65% the size of his neighbors, and unlike them, no comfort complaints. And no setback, which dehumidifies so well that his wife complains if summer setting is below 75.