A flat oval duct system is not all flat oval. A lot of it is actually round spiral duct. No one contests that round spiral duct is less expensive and more efficient than rectangular duct. Most flat oval duct systems are only flat oval in the major trunks. Those branch fittings are usually round fittings – efficient round taps manifolded to the flat oval trunks. The “flat oval” in your duct system is mostly straight spiral pipe, elbows, and transitions. This is where you can really start to save money.