A bit of light at the end of the Outlook tunnel!!! (Sorry guys I'll shut up about Outlook soon but it is intimately tied up with desktop search engines for me.).
I dropped an email to the developers of Neo Pro and got a response within a couple of hours (pretty good). The upshot is that they have not experienced Neo corrupting PST files but agreed that the MAPI interface may be the source of the problem. Their suggestion was to exit Outlook and then run the utility FIXMAPI.EXE (on my system there are copies in C:\WINDOWS\system32 and C:\WINDOWS\system32\dllcache) and finally reboot.
FIXMAPI doesn't appear to do anything (and I'd never heard of it before) but having done as they suggested I can now pound away at my PST file within Outlook and within Neo with no errors cropping up in the PST file as a result.
This is a bit of a result for me.
Regarding slugish behaviour - I used to experience that in Outlook XP but I don't experience it in Outlook 2003. Similarly I used to experience freezes when it downloaded email in Outlook XP but the only time I occasionally experience problems in Outlook 2003 is when I sync Hotmail folders (which I now do rarely). Maybe it would be worth your while uninstalling Outlook completely and reinstalling it again (your plugins, data and settings are retained).
The other approach is to reduce the size of your mail files - probably a good idea anyway for data security purposes. You can archive off emails using dates as a criteria and then have Outlook open multiple PST files so all your data is still instantly available and searchable but your active PST file (with the current Inbox) can be kept small.
I must admit I have spent part of today playing with Thunderbird and with a few addins (esp. Lightning) it is really quite usable as a calendar and task manager - and can do something Outlook doesn't - it can display Tasks in the calendar. The major drawback of Thunderbird for me is the lack of a simple method to archive your old emails and still easily access them within Thunderbird. It does integrate quite nicely with POPfile (you can display the extra header fields POPfile generates using an addin which means you can process POPfile (re)classification quickly from within Thunderbird). There are addins to to access Hotmail and Yahoo mail (and others) via POP which seem to work pretty well (although fiddly and unintuitive to set up). It has sensible methods for dealing with images in HTML mail so that you can trust mail for a single email or from a spefic address. Thunderbird's rules are adequate (though not as comprehensive as Outlook). All in all if you need a free email client Thunderbird is pretty darn good.
Strangely the calendar is supposed to import Outlook calendars as CSV files but I'm damned if I can get it to work.
Here are the extensions I was using:

For anyone that isn't aware Thunderbird gives USENET access and can be set as the default system news reader. I must admit I found the interface for the usenet reader rather clunky and a bit disorganised and confusing.
It also icludes an RSS reader too.