Currently, the licensing fees demanded by Audi make the manfuacturer an unacceptable risk for Tamiya. You'll note that Audi has won Le Mans 3 years in a row and we have yet to see an R8R from Tamiya.
Revell sells a 1:24th cast metal Audi TT coupe which could be modified (using Museum Collection's Hot Wheels/Olive Garden F355 transkit's wheels), but you'd have to create the decals on your own.
I've got some great pics of the bare CF test version I would love to add to my Porsche GT1 and Nissan R390 test mules cars...
hth,
Bill
:bandit: