Let's talk about large mailbox performance, one of my favourite topics! exchange has been getting better and better at servicing larger mailboxes over the years. Since exchange 2007 sp1 we've been able to design larger, more redundant configurations on cheaper storage, however the client experience has lagged somewhat.
Since Office 2007 SP2 my personal experience has been dramatically improved. I deal with several mail accounts in my outlook profile on a daily basis which include Exchange OST and POP/other mail to PST. After an initial upgrade the performance for both offline files and PST's has improved remarkably.
This improvement is just in time as well. Seeing the performance improvements in exchange 2010 on slow disks – 91% at this stage compared to exchange 2003 – were set for much larger mailboxes being the order of the day. I'm not only talking of the occasional power user who may have an Exchange mailbox in excess of 2GB, but the entire user base.
Irrespective of the version of Exchange or how large OR small your mailboxes may be, Outlook 2007 SP2 has brought much required improvements on PST and OST performance.