If you haven't heard already, we have released a Pre-SP2 hotfix that help improve Outlook performance and responsiveness in a big way. Here is an excerpt from the article.
Performance and responsiveness are key concerns for all our customers. That is why we made the large performance tuning and optimization changes that are included in Office suite Service Pack 2 (SP2).
Outlook 2007 SP2 delivers performance improvements in four major areas:
Before you go out applying this on your machine, you need to be warned of the first startup experience as we rebuild the tables and indexes in your OST. If you have a large OST, this is going to take some time, but I can tell you that the wait is well worth it. It is actually an entirely new experience at least for me anyway's since I have a good deal of email item counts in my folders. Switching between folders with large item counts is no longer painful and this hotfix provides immediate viewing of these folders.
Grab the hotfix from the following article:
Description of the Outlook 2007 hotfix package (Outlook.msp): February 24, 2009 http://support.microsoft.com/kb/961752
Check out the plethora of improvements in this release in the following article as there are many.
Outlook 2007 improvements in the February 2009 cumulative update http://support.microsoft.com/kb/968009
Hope this helps tame some of the larger mailboxes that you have.