Reduce the load before chasing a rating
The HVAC system does not create the home's heating and cooling load; the building does. Heat moves through walls, ceilings, windows and floors, while uncontrolled air leakage carries heat and moisture across the envelope.
Air sealing and insulation can reduce the amount of heating or cooling required, improve comfort and sometimes change the capacity needed when equipment is eventually replaced.
Envelope improvements should also consider ventilation, moisture and combustion safety rather than indiscriminately sealing every opening.