It's because of the extra heat mateWhy would it make any difference for track use? They don't take any load.
Looks like you will have to go with plasitc rings as there's no aftermarket metal ones out there in these sizes. You could try contacting OZ directly as I know my OZ's on the MR2 came with factory fit alloy rings as they are OZ stamped.
@perazzi Did you ever find some metal spigot rings? I've been looking for some 67 to 60.1 but both the versions I've tried so far have been too deep for my OZ wheels.