Todd,
There is really not enough info in your post to give you a complete answer. But I will start by saying that performance is a complex animal. Regardless of how powerful your AS400 is, there is a limit to what it can process. An AS400 that is optimally tuned for EnterpriseOne will give consistent interactive performance for applications while running a consistent number and type of batch processes. Note how I am using the word "consistent" and not "acceptable" or "good". My take on performance may be cynical but I have viewed performance tuning as a way to make sure that all users, both batch and interactive are equally unhappy. (That is a bit tongue-in-cheek but it reflects that users are rarely happy. We all want subsecond performance for our 1 million row financial reports, don't we?
)
The AS400 is nice in that you can prioritize your batch load and with a little work you can separate long running, resource-intensive UBE's from quick, low resource jobs. You can do this with other platforms as well but the AS400 makes the process really easy to do.
Now, after having tuned your system in this way if you are still consistently over 80% then your only solution is to upgrade your processor capacity or reduce your workload. It is the old 10 pounds in a 5 pound bag analogy. Something has got to give, either the stuff going into the bag or the bag itself.
Take a look at these links for a good place to start learning about AS400 work management. Like all IBM manuals there is a lot in there. Unfortunately despite what the IBM marketing will have you believe about the AS400 being a magic black box, tuning is still required and takes some effort.
This link points to a list of performance tuning resources
http://publib.boulder.ibm.com/iseries/v5r1/ic2924/info/rzahx/rzahxperfbooks.htm
This link points to the Work Management manual which I feel is the best place to find out about managing your batch and interactive workload.
http://publib.boulder.ibm.com/iseries/v5r1/ic2924/books/c4153063.pdf
I hope this helps.
Regards,