I can't believe I just lost the link but I will try to find it. Here is the jist:
A retired HVAC guy teaches for free at a university. A study was funded to evaluate various manufacturer's equipment. He said that 16 SEER was achievable in an ideal lab setting but next to impossible to obtain in real life. He said that if the refrigerant charge was slightly high or low SEER would "drop like a rock". He said most HVAC companies do not have the equipment or trained personnel required to get the precise refrigerant charges to obtain 16 SEER. Is this guy full of baloney?
He also said that most homeowners would never get a payback on the high capital cost of 16+ SEER units. He said "If you want to save energy buy 16 SEER. If you want to save money buy 13 SEER."
This is important to me because I need 16 SEER to get the fed Tax Credit, but 16 SEER may cost more than I get in the tax credit.