Tough question. I would have gone with the two separate systems sized properly from an accurate heat load analysis. I encourage people to have the house energy modeled before construction that way you can make informed decisions about construction details. If you talked your builder into increasing the AC size he probably doesn't know what size should be installed. Freezing coils is an airflow issue if the unit is charged properly but you shouldn't need to go below 72 IMO. Correct air flow can only be determine by a room by room load calc and then testing the system to verify performance. Was this done? The #'s look like a 5 ton 13 SEER builder grade heat pump.