Have you had any luck in your research?
We ourselves took a big performance hit when we upgraded to 4.x, although we made several new deployments at the same time that could be part of our decreased performance too.
Here are a couple of links you might have already seen - if not, they are helpful.
A JBossian blog posting on rmi gcinterval and also stack size:
Sun's JBoss On Solaris Best Practices
Here's an extensive article on slimming your jboss installation. It starts with tuning tips
And you probably know this one too, but just in case:
Along with this:
If you haven't yet already, check out
I use that on production boxes instead of verbosegc
And jmap with the -heap option is awesome. The -histo option is pretty cool too.