Currently we consider the following duct materials: regular (galvanized), aluminum, PVC coated, black steel, and stainless steel. We don't differentiate light and heavy gauges for the moment, but it can change later.
Unfortunately we can't make all the materials with "true Revit" functionality like defining a duct type for every material. It not only multiplies a number of duct types, but also requires to duplicate and assign duct fitting families for every material. I found it is not realistic for now. Duct and duct fitting materials are driven by the "Duct Material" text parameter (actually we are using predefined combinations of the duct material and allowed external and internal lining). Ducts in plan view are hatched according the material. Also we create a duct material take-off report with Revit schedule.
I believe, in your example the branch and the main 'solids' should not be a problem because the Family should be properly designed which means it should be one single solid created with the "Join Geometry" command. It would have true 3D geometry resulting proper surface area calculation. I don't see a problem here, but there is a problem with a tap takeoff fittings from round and oval ducts. Revit doesn't cut the main duct 'solid' from the tap. If one day Revit could do that then it would be no obstacle for true fitting area calculations.
We as consulting family don't care much about the duct area calculation precision. I think the proposed mathematics works well for vast majority of the fittings. Even with the described imperfectness the fitting area calculation would be absolutely acceptable for us. An approximate result is much better than nothing.