I have made a suggested modification to
Bug 17382 – Can zimbra SA work with an External SA? to address the issues of customizing antispam settings. The more I think about it, I believe
Bug 3870 – per user Spam Assassin score is a different issue and while I support it too, I don't necessarily see it as appropriate to combine it with the other two referenced above.
But I do recommend that you all vote for the above two bugs and see what further changes you might suggest on either one.