After scouring the net for about six hours I came across this extremely useful tool which is of great help when trying to transfer mail from Outlook on Windows to Mail.app. It's called MailRelayer and acts as an IMAP server for Outlook, thereby making your entire PST file's contents available to another machine.
I transferred about 1,900 messages (250MB) in about 45 minutes over LAN, so the speed isn't too bad. Another bonus is that messages transfer without attachments being TNEF-encoded.
A 30 day trial can be downloaded from www.mailrelayer.com.