We are eagerly waiting for the release date of Exchange 2013 CU5 which could fix transport agents not loaded after Sp1 upgrade as mentioned in KB2938053, Shared mailboxes sent items are not saved in the Sent Items folder of the shared mailbox and it gets stored in drafts folder of primary mailbox.
I just happened to read the latest Tech-net blog posted by Ross Smith which mentioned about Changes in OAB from Exchange 2013 CU5.
The main highlights are
1) Single OAB Generation Mailbox per site. Which stops multiple OAB download instances from multiple OAB generation mailboxes located in same site?
2) Having one OAB instance per site which stops multiple downloads of OAB files.
3)We can Specify OAB generating Mailbox.
Read more from Source Tech-net Blog:
Hope this information will be helpful in planning for CU5 upgrade .