Thanks for the reply.
I couldn't find any information about the GermanStemmer/GermanStemFilter in the elasticsearch documenation.
(Is there any reference to this stem filter in the official documentation?)
I tried the Cistem stemmer via a custum plugin, too.
For example the words Wunsch, Wünsche, gewünscht and wünschen where all reduced to the same stem (wunsch). The Caumann stemmer didn't do that.
But both stemmers seem to have their limitations. So for example Freund and Freunde where stemmed to a different stems than Freundin and Freundinnen.