What is your house like, There are always ways to improve the envelope before you upgrade the system. Your actually putting the cart before the horse.
My opinion is to do an energy audit on your home before you upgrade. I have experience in this area and recommend it to everyone before you do a mechanical upgrade. If you do it this way, you save thousands up front that year. To many things go wrong in a home over time, insulation settles or was never installed in some areas ,ductwork works loose, systems get out of wack, the list goes on. I enter a home with an Infrared camera and see the heat leaving building which have an energy star rating.
Many systems are oversized already because of the envelope not able to support a smaller system. The HVAC company does not have faith in insulators, been like that for years. I don't blame em either.
I don't know how many more ways to say it...you need an evaluation from someone OTHER than a heating guy.
Sorry if I'm being forward, but I see it done in the wrong order ALL the time. We need to change that.
Envelope first, system sized second.
Sorry HVAC guy's.