Optimum level for Exchange Mailbox Archiving Task

It might. At least it might mean that you'll get through the users in the time window you've got. Remember that the items per pass just mean how many items will be archived 'in that pass'.  Once all the users are done, if there is still time left in the schedule, another pass will be done, trying to archive up to that same number again.  Another option of course, is to get more horsepower in the CASes.. and have you had complaints? If not, leave it at high CPU.. that's what the CASes are there for...