Update: I just spoke with my friend who, as I said, is not just a Jag mechanic, but is a Master of masters Jag mechanic said the spec is...drum roll....there is no spec. That's right friends, there was never a spec published for the EType rotor thickness. Basically he said that unless they are really thin (more than 2mm less than a new rotor, they are probably ok. However, he also said that if they have to take more than 10 thousanths off in order to true them, you should probably disguard them and buy new. The good news is, new rotors are very reeasonable (about $35-$45 each). If you have them off and there is any doubt, you might want to just consider putting new one on anyway.
Cheers,
Basil