In short: you can probably do better than 3-5 spams per week with SA.
The big problem is the entire thing is a beast to configure with all the documentation of a Babylonian cuneiform stone tablet.
I'm more curious about the opposite metric: how many non-spam emails a week arent getting delivered to you? Because that seems to be the real flaw in spamassassin: the false positive rate.
And the spamassassin users don't usually have much visibility into this, so when emails don't get to them they just blame the sender.
My false-positive rate is very low, maybe a couple per month. However, I can predict with a high degree of accuracy when a piece of email is likely to land in the spam folder. Things like confirmation emails, registration emails, etc. are guaranteed to land in the spam folder. It's pretty hard for any system to accommodate those without allowing spam to get by.
That's fine by me, though, because I know when to check my spam folder.
I also have dovecot set to learn Ham every time I file an email from the inbox to a folder for good measure.
1 spam a month would be .018% of emails and 5 x 4 spams a month would be .364% of emails
So I would have gotten about .346% more spam based on the number of emails. In reality, because I don't see all of the mails, it's less. Is a touch more than a third of a percent a 'big difference'? YMMV.