Mitchell,
Using Compliance Accelerator against user archives is not very efficient as the data to be collected could be days or months old, depending on the mailbox archiving policy settings. Needed items could be missing if any user deletes items prior to them being archived. Also, CA would have to search against the index volumes for each user archive using the SMTP addresses and / or Display Names for the employees that are determined to be needed as Monitored Employees.
CA works best against journal archiving with the Journal Connector in place and properly configured to tag messages based on the Department ID of any and all Departments to which the Monitored Employees that are message authors or recipients are members. The Department tagging is a much more efficient means of obtaining data for review in CA, both from a Random Sampling and searching perspective, as using the numberic Department ID is preferred for data gathering (less SQL and EV Indexing Server overhead needed). Plus, journal archiving does not allow any user deletion of messages prior to archiving as user's don't have access to the journal mailbox.
If the cutomer's need is to just search against archived messages for some litigation purpose, Discovery Accelerator or Clearwell are better choices as the can have a more free form search criteria. Remember that CA must have Departments with Monitored Employees. This leads to the CA search criteria needing to specify message direction into and / or out of the Department's Monitored Employees.
Now, with all of that said, your customer will get better overall performance if they have an accelerator server (CA, DA, or Clearwell) in each site. If they have a really 'fat' network pipe between the sites, they may be able to get by with one central CA server, but it having to access index volumes across the WAN for searches and storage for exports could be very problematic (i.e., excessive latency that could lead to time outs).
