Here is a good approach.
Ask your AC contractor to perform a heat load calculation, and compare the required CFM for each area of the home to the available ductwork. That immediately determines a couple of things:
1) The needed capacity of the system, according to the design temperature for your locality, and
2) The ability of the system to move the cooled or heated air to the places where it is needed.
One path might be a remediation of ductwork to properly move the air, and an indoor unit that matches the outdoor unit with a sufficient blower to move the air through the corrected ducts.