Tomcat is the reference implementation for the jsp and servlet specs. So new features get added pretty quickly. The new tomcat series (4.1.x) uses rewritten http connectors and jasper jsp engine. I do see significant performance improvements using it as opposed to 4.0.x especially in running jsp pages.
You should probably also look at Jetty which is the default webserver shipped with Jboss. It seems better integrated into Jboss, and I have seen some mention in these forums about adding Apache in front of Jetty.