A few suggestions:
server side mail rules , configurable via squirrelmail (this requires maildrop i think, i'm still strugling with maildrop on ubuntu) - this is the feature i'm really trying hard to get working.
postfixadmin - really comes in handy when administration is required
mysql change - move from mysql tcp access to the the sock , this is fairly easy to do, and safer.
Question : what are the tables for when setting up squirellmail ?
I dindn't follow you're guide, but it's very good. I followed similar one's, but i guessed you already read them, since the similarities are remarkable
Anyways, great job. If you need help on the proposed addons, talk to me, i can lend you some words and steps to configure.