I know about 2, but there are IMO 3 or more. There is one in Berlin at the Charité Hospital - it used to be Dr Gross but he seems to no longer do this himself. The other one was in Stuttgart at the hospital there, I believe they still do this there. It may be that there is one in Dachau who does it as well, I had the adress here somehwere as this is closest to me.
But as you say, the results matter and those always warn you a lot about possible hoarseness and breathiness. I heard no good result of them yet, but my experience there is limited, maybe thosw with good results dont talk about this. They will not provide voice samples, except in some cases if you ask them for it at a personal consultation.
If you want, I can find out more details, but as I said, I would not recommend them. Consider this: I could with near certainty get a insurance covered glottoplasty with one of them. I would have to pay nothing as they are in my home country and I can provide enough compelling statements that my voice is bad and needs to be fixed. Yet here I am, saving money every month just to fly half around the world to Korea if I want to do this. Its results that count...not money or travel distance