I am not sure if the Google apps filter shares the same engine as the gmail spam filter but here are my stats.
I have an email address (using the google apps) that is not published anywhere, any email in there is spam.
In the inbox I had 52 emails
In the spam folder I have 1689 (including the 52, I just put in there)
So of all the emails I got there 52 slipped through that gives a "made it to the inbox" ratio of 3%