Most of the research in this area has focused on genetically modified mice. So wether the mice are altered as myostatin knock-outs, or to inhibit myostatin (through increased expression of say forstatin), the lab results came about in mice born with these characteristics.
I am not aware of any studies that demonstrate substances that can transform animals of normal genetics through inhibition of myostatin, say in adulthood. Well, it is not to say it is not possible, I just think that if it were as easy as that, we would have seen some studies or something more significant from the major drug companies. I would bet they are working on this but I just have a hard time believing that Biotest beat them to it.
And plus Biotest even says that the effects will be rather slow-acting (over say a year), so they are basically admitting this is just an experiment (since they have not done any real tests themselves - just rushed the thing to market based on some lab experiments). I'll even go so far as to say they know it will not work, if it were as powerful as they say, there would be too many risks in rushing it to the consumer.
Just think, what would it take to feed the supplement in high amounts to some mice to see the effects?