IMAP support for Exchange is shit. You don't want it. Also there is security issues with trying to get IMAP working and requires extra work and vigilance on the part of admins. It's no surprise that admins are not wanting to support it. I wouldn't want to take a kerberos domain and try to degrade it's security by adding IMAP for Linux compatibility.
OWA is also shit. The web interface for Exchange is unreliable and locks up continuously for myself and anybody that uses it. It's just very bad. That's the web interface _being_used_as_a_web_interface_ and not a hack to get Evolution to pretend it has Exchange compatibility. It's not surprising to me that Exchange support in Evolution makes Evolution run like crap. You can't make a silk purse from a sow's ear. OWA by itself is shit, expecting OWA + Evolution to be anything else is just pure wishful thinking.
What you want is native MAPI support. This means using the client support that Exchange has that is, you know, is designed to be used by clients.
This also means that you have AD intergration, some SAMBA 4 stuff, and the whole ten yards. A Outlook replacement for the Linux desktop, not a second-class imap pretender (after all, if Exchange was just about email then it would be a easy kill for the OSS community) or alternative front-end to the web interface.
Of course this required functionality is not yet present in Evolution. It should be in Gnome 2.26 though and is probably one of the more important features for the Gnome desktop in a long time.
Thank goodness for OpenChange (not to be confused with other projects like Open Exchange) and Samba4 for providing this sort of minimal functionality that is expected to be present in modern business desktops.