Yes, there are a million things that you could change that affect the performance. For some tuning tips see http://wiki.jboss.org/wiki/Wiki.jsp?page=SPECjAppServer2002Tuning. Though the wiki article is specific to tuning that particular benchmark, the general principles apply to all applications, and the article at least points out many of the settings that you can change.
Also, Andy Miller gave a performance tunign presentation at JBossWorld. You can get it at http://www.jbossworld.com/downloads/presentations/. His presentation was on Thursday.
Finally, I do have another detailed reference but it is not free. I can supply the URL if you like.
I read that the supplied jboss binaries are a debug version and one is able to recompile the code without debug and gain performance.
I tried to compile the sources but failed using 1.6. Seems to be a known issue and using 1.5 supposedly works fine.
If anyone has a non-debug 1.5 build, maybe they could share it?