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Sounds like your contractors are feeding you a line of crap. The newer units produce the same amount of ac as the old ones. In otherwords if you had a true 5ton (60,000 btu) unit and it cooled fine, a new 60,000 btu 5 ton should cool equally well just more efficiently.Hopefully Trane will follow up with you and get these slackers do find out what the problem is.
The 20 degree is a good rule of thumb, but if you had a five ton previously and new unit is a 5 ton, you should be able to get equally cooling.
Do you have a thermometer to check the temp difference between return ands supply air?