OZ1,
we have had experience with auditing Security, HR/Payroll and Purchasing(Contract Admin and FPAs) in MIMS 4.1. Here are some notes:
1. some areas in MIMS have intrinsic auditing on the normal tables, ie PO history of changes msf222, Contract History msf394-397, etc so possibly you do not need to implement Audit Trail unless you have specific requirements
2. Audit Conditions: can be specified as Cobol conditions on the audit plan file line, eg we use following entry to audit FPA Group and Agreement only (to exclude FPA Items from auditing):
MSO254 MSF254 IF (MSF254-FPA-ITEM-NO=ZEROS) AND (MSF254-FPA-REC-TYPE="F"

You ask about limiting auditing to particular userid and actions, I would suggest to use the request parameters of audit reports and not the audit plan file for that purpose, ie audit all users & actions but filter at reporting time. Note that if audit records have not been collected by MIMS because you restricted to a particular user, if the need arises in the future to track on somebody else's past actions it will be impossible to report.
3. Audit reports:All our audit reports are custom RDL's usually based on the auto-generated MSZ* reports. In the past we had instances when some MSZ* programs would not compile or would generate wrong output especially with OCCURS fields.
MSZ* are very technical and we had to create user-friendly output.
4. Records Volumes: you may need to monitor together with your database administrator. We have no reports so far on adverse impact on performance. We would run MSB033 periodically but once you ran MSB034 all your audit records are gone, so make sure hard-copy reports have been archived or better still a copy/extract of MSF034 is available. Until recently (Oct/00) redundant start/end transactions records were being written by MIMS 4.1 so if you run this version ask Mincom for the latest patch to macros
5. We had some issues with MSB033 in multi-district environments; test carefully what happens with msf033a/b as you run MSB033 with or without the district code field spaced out on MSM080B
6. Be aware audit records are available for reporting only after MSB033 ran, and then possibly with a 2 days delay if MSB033 runs daily and switches operation from msf033a to msf033b
7. after re-compile of program with the /audit option be aware audit records colection may not start straight away, you need to ensure the new executables are used and one way of doing that is to shutdown and re-start Mims.