You say you have a horrible performance issue, not issues so is this just one job or the entire system? If it is a single job you can pay IBM to ‘lend’ you their Job Watcher(part of iDoctor) and they can analyse jobs pretty well. If you added PTFs lately and run large SQL jobs its not unusual for PTFs to affect query and query optimizer functions. Do you have the IOPLESS disk cards for the Shark? If you are old IOPs you’ll never get the performance out them and I still don’t believe they will perform as well as internals.
I know of no third party performace tools for the iSeries, but give us more info and we might can point you to some solution.