I've had the same issue on my '01 S8. I did think of the metal Gruven items, but I would imagine the OEM parts are plastic for a reason. To protect the internals.
If the shafts are seized, the plastic arms would break. If these arms are metal, something else would break (internally, and more expensive). In my opinion, the aluminium arms would only serve to mask an inherent problem. Don't under estimate the force those vacuum units exert. If the shafts or flaps break internally, the engine would swallow the bits.
I had 20 made for me, from a less-brittle plastic, as durable as the originals. These too would break, if the shafts were seized in the bushes. These are available for any interested parties.
http://forum.a8parts.co.uk/showthread.php?t=4210