While this isn't perfect, fewer than 1 spam per month has made it through to any of the lists we host in the last year (at least one of which I approved accidentally :-).
I think it should be noted that this is probably the first time I've seen a spam on the centos list that I can remember. And I've been subscribed here off and on for a couple years...
The system you're using does sound like a solid one though.