Flex duct definitely has more resistance than round sheet metal of the
same diameter. However, the difference is not so much that simply
replacing your flex with metal will likely solve your problems. There
are a lot of issues -- exactly where these ducts are tied into the main
ductwok, and also how many CFM your system is putting out. Check with
other a/c contractors, but I seriously doubt that merely using flex duct
is your problem. Larry