So -- when doing a migration of about any non-trivial size into 2010 you're going to have to set a new throttling policy here called "SuPolicy":
New-ThrottlingPolicy SuPolicy
Set-ThrottlingPolicy SuPolicy -RCAMaxConcurrency $null
-RCAPercentTimeInAD $null
-RCAPercentTimeInCAS $null
-RCAPercentTimeInMailboxRPC $null
-EWSMaxConcurrency $null
-EWSPercentTimeInAD $null
-EWSPercentTimeInCAS $null
-EWSPercentTimeInMailboxRPC $null
-EWSMaxSubscriptions $null
-EWSFastSearchTimeoutInSeconds $null
-EWSFindCountLimit $null
Set-Mailbox myserviceaccount@mydomain.com -ThrottlingPolicy SuPolicy