Hi Charles,
"I've seen one post on this circle from an Amadeus owner who added Herbie's mat and clamp to his rig with great results. I've also been told that none of these things should be used. Does the added weight of aftermarket mats and clamps pose a problem for the motor or the spider web filament that serves as the drive belt on this TT?"
in a word, no. and definitely a no to the clamp (although I've never tried one with it, and unless BF starts using one, I never will.) besides, why put something between you and playing a record? isn't it great not having to use one?
Granite seems to be a british thing, and while I'm all about the british, I wouldn't use anything that's going to deaden the music. A good support usually has a good shelf already.