From my experiences I can tell that it's definately worth to look into installing IBM's JVM. I experienced a huge speed increase (maybe twice) while stress-testing my application.
Im also searching for more pointers to improve app-performance. You might look into using opensymphony software (http://www.opensymphony.com) to speed things up.
One basic thing you might want to check would be the Tomcat configuration. Out of the box the settings don't support that much of a load. Try tweaking the minProcessors, maxProcessors, and acceptCount properties in the tomcat-service.xml file. I've forgotten to tweak those before and JBoss/Tomcat starts tripping all over itself way before it ought to.
Just as a general Good Idea (TM) set enableLookups to false. Doing all the reverse lookups is a huge bottleneck that you don't want to bother with. Might not be affecting you if you're on a local network with a single load client, but down the line...
would you mind posting these Tomcat performance optimization tips to the FAQ forum?