What does the code say on what temp you have to plan a HVAC unit for. I can tell you that I took temps in every vent in the house. I am loosing 4 degree's from the return air duct to the return air plenum. Most of the vents that are away from the plenum are loosing any where from 6-7 degree's. It is coming out at 62-66 degree's at some vents. The air in the plenum after it leave's the evaporator coil is 59.9 degree's.
6-7 degrees? That seems like a lot!
If you were getting a 20 degree temp drop across the Evaporator, a 6 degree temp drop from evaporator to the register would mean you're losing 30% of your cooling capacity to that register. No wonder you're not able to get the temp down. You need to find out if your ductwork is insulated and if the joints were sealed.