Well, here is the deal, you will not find any JBoss benchmarks, because JBoss doesn't believe in them because they are not reflective of what you will actually see, each deployment is different, so looking at any benchmark to your application will always be apples to oranges. And obviously any benchmarks created by competition companies will always skew to their server. So you cannot trust any of that information. Besides 56.2% of all statistics are made up.
So, you need to set up a small machine and do some tests of your own. It depends on heap size, your size of your sessions, pool sizing of Connection pools, EJB pools,
Servlet Pools, amount of data coming through the wire. There are lots to look at.
Good Luck and try getting the Apress book called
Java EE 5 Performance and Management.
Mark