I wanted to buy a super efficient multi-stage or atleast 2 stage heat pump and got three quotes but then another contractor came in and refused to quote or install that, saying upstairs master would never get cool enough, and I needed single-stage (american standard silver 16 he recommended). It's a brick 1948 colonial with recent insulation but poor duct work. Someone asked this or very similar 7 months ago-- the comments were useful but varied and I wonder if anyone one has any other thoughts - is there a test that can be done to see? Currently have a 35 yr old furnace and 25 year old air conditioner and upstairs master is marginal in summer- I try shutting vents off downstairs sometimes and that helps.
There really is no cure for inadequate duct work that involves making use of the existing ducts. If one looks at the fine print of super efficient systems, there is always a qualification that the units must be installed correctly to achieve the rated performance. Good duct work is a big part of correct installation. If the duct work is marginally bad, adding an inline booster fan to push more airflow where it is needed might work, but that kind of solution reduces both efficiency and reliability.
Ductless mini-splits are sometimes a good option where existing ducts are not usable. Some mini-splits offer the highest efficiencies available and they also are equipped with inverters that provide variable capacity control that adjusts on the fly. Also, different models of the indoor portion of mini-splits are available to satisfy different situations.