Do you by any chance work with Rajesh Gupta? See: https://community.jboss.org/message/732025#732025
Alos, the more load you put on the system the more threads that will be used to handle the load. This explains the CPU utilization increase under higher load (or when the number of threads increases). Usually there is a "sweet spot" for the number of threads as compared the to number of CPUs you have. Then there is also processor affinity which can play an importanmt role - somtimes the JVM behaves better if it is limited to 2 or 4 CPU even if you have 8 available.