When selecting the air conditioning and heating equipment needed to provide the comfort and energy savings that is listed on either the condensing unit [outside unit] or the airhandler [inside unit] the two pieces need to be matched per the manufactures recommendations.If not you will not get the performance you were told. That is SEER rating or BTU rating.

A study done by the Alabama Energy Commission concluded these results:

        1.Up to 40% reduced capacity[BTU's] in cooling meaning system may not be able to keep up with thermostat setting.

        2.Up to 8% reduced capacity in heating resulting in strip heat coming on earlier.

        3.Up to 40% reduced efficiency in cooling resulting in higher power bills.

        4.Up to 60% reduced efficiency in heating resulting in higher power bills.

        5.System charging becomes critical since the charge might seem correct for one condition but flood on another. 

        6.A thermostatic expansion valve will not solve the charge problem for all cases since the sensible heat and subcooling can change dramatically with conditions.