First of all, assuming the duct is in attic and the ductwork is original 1975ish then it is not properly sealed or insulated if you seal it and reinsulate the duct, provided it is sized correctly, then 2 tons will probably work well, the existing 2.5 ton unit is probably not delivering 2.5 tons due to age and mismatch and you are probably losing 25+% due to duct loss. If you install a high eff unit on a leaky duct system you will not see the rated efficiency and often it intensifies hot/cold spots and humidity problems rather than solving them. The ductwork is critical for a long lasting efficient system.