I did some further experimentation with the Proxy graphics and MEP duct/fittings. It seems that MEP 2013 set to PROXYGRAPHICS=1 can save pretty much every native MEP duct/fitting to display in previous versions. We are having real big issues though showing some of the custom MvParts and duct fittings created for our own catalog using the content builder. Since we are members of ADN, I am putting in a support request about these items. I will see what they say and keep this thread up to date.
what Autodesk is doing by changing the file format so frequently is forcing the consumers to stay up to date and upgrade regularly. it seems to me there is no need to change the file formats so much and just leave somethings alone. It disgusts me that I have to use this program at all and further disgusts me that my company has to pay for it. Sometimes Autodesk should listen to their consumers and leave some stuff alone instead of trying to create better but instead creating more headaches.