One year I was responsible for supervising the repair of 2500 heat pumps in many states of the USA. The greatest installation problem was the lack of adequately-sized ductwork. The Probability is very high that your duct is already too small. Small ductwork creates high gas pressures in the machinery and reduces the amount of heat produced - makes the machine less efficient than it is.
You can make that section of ductwork shallower if you make that section of ductwork wider to accomodate at least the same amount of air without creating more resistance. A sheetmetal duct-maker can do this for a small expense. A hundred or so spent now is better than a new compressor and a cold house expense later.