Would it be a perfomance issue when I set generate_statistics to true? Or would this improve performance of NHibernate? Should I always set this to true - also in production system?
At the production you should not enable it. As ayende says "It is not really recommended, it results in locks to update the stats all too often." http://groups.google.com/group/nhprof/browse_thread/thread/841fbc918418041b