I found the Solution - Thank you @arootbeer for the hints!
Instead of wrapping the XMLDeserializer, you can also use the 'RestRequest.OnBeforeDeserialization' event from #RESTSharp. So you just need to insert something like this after the new RestRequest() (see my initial code example) and then it works perfect!
request.OnBeforeDeserialization = resp =>
{
//remove the first ByteOrderMark
//see: http://stackoverflow.com/questions/19663100/restsharp-has-problems-deserializing-xml-including-byte-order-mark
string byteOrderMarkUtf8 = Encoding.UTF8.GetString(Encoding.UTF8.GetPreamble());
if (resp.Content.StartsWith(byteOrderMarkUtf8))
resp.Content = resp.Content.Remove(0, byteOrderMarkUtf8.Length);
};