Outlook 2011 Folder Item Count Recommendations

Recently a user asked what the limits were for good performance in Outlook 2011.  Checking around the various support articles I found (italics mine):

• Microsoft KBase Article 905803, “Outlook users experience poor performance when they work with a folder that contains many items on a server that is running Exchange Server“
  “We recommend that you maintain a range of 2,500 to 5,000 items in a folder, depending on the capacity of the Exchange Server environment. Additionally, you can create more top-level folders or create subfolders under Inbox, Sent Items, and Deleted Items. When you do this, the costs that are associated with index creation are greatly reduced, if the number of items in any one folder does not exceed 5,000.”
• Microsoft article “Best practices for Outlook for Mac 2011“
  “If this folder becomes too large (10,000 items or more), Outlook for Mac 2011 might become slow when switching to this folder.”
• Microsoft article “How do I improve the performance of Outlook for Mac 2011?“
  “Keep a maximum of 20,000 items each in the Inbox and Sent Items folders (includes the folders and subfolders). For optimal performance, keeping less than 5000 items each in calendar, contacts, tasks, and notes folders is highly recommended.”

So the number of items is in question and also whether or not subfolders count toward that number.  I contacted Microsoft support and asked which answers were correct–here is the response:

• “I would recommend keeping the Inbox, Sent and Deleted items folders below 10,000 items if possible.”
• “Keep a maximum of 20,000 items each in the Inbox and Sent Items folders (includes the folders and subfolders).”
• “The Calendar, Contacts and Tasks should stay below 5,000 if possible.”

Note that these recommendations were easier for Microsoft support since my company uses Office 365 Hosted Exchange and thus the Exchange environment was known by support.  Different Exchange servers may not be able to support this sort of load and may require lower item counts.