1) Can It support mod-cluster ? I am really looking forward to using mod-cluster.
Yep. Mod_cluster httpd part works with Tomcat, JBoss Web, AS and with/without clustering the JAVA part.
2) It looks like i'd need to front these with apache/httpd since I want to use mod-jk/mod-cluster, any issue with this?
Should be no problem.
3)How well does this Java webserver perform ?
4)Does it scale good in Linux platforms ?
You have no problems with that we have jbossnative to speed up IO and java scales well on Linux additionally when using httpd as front-end you could build different clusters for your applications.