About the damage...
Growling does not use the vocal cords. Above the vocal cords are 2 folds of muscle called false cords, and when you growl, these close. It's called a ventricular voice... or something...
The reason you get a sore voice after, is because once you close the false cords, it takes a while for them to retract, and you can't get much air through to speak. And I think they push on the vocal cords which can cause nodes, which if you get, you will not be alowed to speak for months until they are gone.
Growling, in the long run, will wreck your voice. All training will do is prolong it.
I stopped growling, and am focusing on my melodic singing, with only occasional growls, as I don't want to wreck my voice.
All of this info is from a voice trainer by the way... so don't bitch to me saying I am wrong.