One of the main benefits of private volumes is fault isolation.  If a disk drive in a private volume set were to fail, you would only need to restore that volume set, rather than doing a complete install of the whole system.  Since you have mirrored arrays, that benefit diminishes in importance.  I don't find system administration to be enough more difficult with private volumes to even consider as a factor.  About the only consideration is creating and deleting groups.  For each group you create you have to do two NEWGROUP commands - one to add the group to the system directory, and one for the volume set directory.  There are command files available on Jazz that automate that extra step for you, if you find that problematic.  Backups and restores are no different with private volume sets.

Yes, you can manipulate class names without a reload.  Others have already provided instructions.

Regarding maintenance of your Nikes, you have three choices:  (1) HP will probably still support them on a time and materials basis, (2) You can get a maintenance agreement with a third-party support organization, or (3) Stock you own set of spares.  In my opinion, the best answer is either number 2 or a combination of 1 and 3.  In other words, have spare drives available to replace any that fail, and pay T&M charges if a more involved repair is needed.

One more thing I want to comment on: Craig characterized XM as a write cache.  I disagree.  XM is essentially a log of physical transactions in progress, whose purpose is to allow the reversal of incomplete transactions in the event of a system failure.  Disc caching happens regardless of whether you have XM enabled or not.  If having multiple instances of XM does anything for performance, it's because of increased parallelism and less frequent checkpoints.

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