Make sure the installing contractor does a "Manual J"/ Heat Load Calculation. I am an HVAC Tech and trust me, this makes a whole lot of difference over the "Oh your house is 2,500 Sq. Ft., you need a 3ton unit." If the unit has been in the house since it was built the capacity needed may have changed due to old windows, things wearing down, heat load changes from trees growing/dying.
Also, if it's just not seeming to cool as well it may be dirty. Take the panel off the inside unit and clean the evap coil and make sure there is not a lot of junk built up on the blower wheel. (Make sure the power is off while doing this) Also clean the outside unit as well. It just may make the difference you need without a whole new unit.
