As well, I too write about this and suggest that folks google "DOE duct leakage" and read for a while. When you think about it, it is stunning that the HVAC trade has done such a poor job in installing systems. The means of proper sealing have been known and available for a very long time. It is so much easier to do at the time of installation because you have unrestricted access. Duct repair is very expensive and difficult work, due primarily to the position of both the person doing it and the position of the connection.

This puts all unitary equipment manuf (Carrier, York, Lennox, Trane, etc) in a very difficult position. They can put all the technology they want into their equipment. They just can't deliver it. Between their technology and your comfort is as the DOE studies have shown an 18-42% leakage rate. So, ER, your 30% average is the number I use as well.

I have gone "Net to the Space" in my heating and cooling system. I pulled the whole ducted system out and took it to the dump and installed Rinnai Energysaver gas DV wall furnaces and mini-split heat pumps. I gained enormous storage space and better comfort at substantial operating savings. Thanks for that link!