Your right, there is some double effort in fighting spam here. And initialy it doesnt work so nice. But when the filter gets seeded with spam/ham it gets better and better. Maby it makes spammassasin obsolete etc. But it still works.
So if it dont work for you, you can still go back to the old situation. I figure thats handy if you have a big ISPConfig system. If your statifiyed with the proxy you might uninstall spamassassin etc. But it wont hurd to have it as a backup.
The essence of a proxy is an inbetween strategie where dirt gets kicked at the port of entance. In this way it can be an entirely different machine too.
If you do not want to have all kind of problems in the begin you just have to run in test mode. Its in the interface. after a week you go to "real" mode.
There is a email Interface so your users can interactively modifiy the whitelists seed the spam filter and notify false positives.
And as Falco said just remove
in /etc/postfix/master.cf and your back where you begon.