I spent a few weeks this past fall trying to configure spamassassin to be useful, but I was getting up to 30 pieces of spam a day and it was getting obnoxious. I configured my setup to use greylisting and it wound up being significantly more effective. I get maybe one piece of junk a week.
I'd like to do that, but greylisting is typically done at the SMTP level -- before the messages are anywhere close to SpamAssassin. Can I ask how you're accomplishing this?
You can run SpamAssassin at SMTP time in the DATA phase. At least, you can with both Exim and Postfix. I assume other MTA's support that too. You can run Greylisting at the same stage after SpamAssassin has been run and before the message has been accepted.
https://help.ubuntu.com/community/PostfixGreylisting