I updated my IMAP stress testing tool to support also APPENDs and print some nice statistics. If you want to try for yourself, it's here:

http://dovecot.org/tools/imaptest.c

I thought about adding some more features in future to it, such as:

• Add a header to each appended message containing MD5 sum of it. When fetching, verify that the MD5 sum is valid.
• Do randomized fetch requests (headers, size, flags, etc.)
• Do randomized search requests
• sort, thread