when replacing old air conditioner4 with high efficiency model, is it necessary to increase the tonnage, i.e. 3 ton to 3 1/2 or 4?

I have just had a new air conditioner installed (3 ton replaced by a 3 ton). The new unit is supposed to be more energy efficient, however, it does not keep the temperature at the desired temperature set on the thermostat, i.e. by 4:00 PM the thermostat is set at 75 degrees but the temperature doesn't get below 80 or 81 degrees. The AC company wants to attribute this to the high temperatures we are experiencing, however, my old AC always was able to cool to the temperature set on the thermostat. A friend told me today that with the "new" freon, you really needed to increase the tonnage of your AC in order to achieve the same cooling efficiency as the old one. I really don't see how it can be more energy efficient when it doesn't ever shut off. Also, how does Lennox compare with Carrier?