I did the same - deleted the Mail folder, all plists, etc and it fixed up my "poor Exchange download" issue immediately.
Before deleting the Library\Mail folder, I copied over my locally downloaded email folder (titled "work emails" to an external location. After restarting Mail and setting up my account, my Exchange email is working. All good so far!
I copy over the "work emails" folder back into Mail and restart Mail. It can see the local email folders, the hierarchy and all but it is not showing any emails in the folder.
VK - can you please help?
Thanks,
Waleed