I have 3 years of old mail saved in a separate mailbox that now contains 12000 messages. They are mostly plain text.
I tried using the “add mailbox to Devonthink” script in the mail scripts menu, but it is extremely slow. It takes nearly a minute to index each message. I even let it run all night and it only archived 900 messages out of 12000.
The speed is limited by Mail 2.0, not by DEVONthink Pro. E.g. it was possible to import up to 50 mails per second using Mail 1.x (part of Mac OS X 10.3.x) but unfortunately Mail 2.0 is slow as hell.
Yes, having developed a new way for DT to archive mail I can say that AppleScript in Mail 2 is severely limping, especially compared to Entourage and PowerMail for instance. (Check the blog of the president for some news related to this.)
There is hope.