> how many non-spam emails a week arent getting delivered to you?

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.

Fastmail user here, so SpamAssassin I assume: virtually no false positives. My GMail spam folder is generally 50/50 false and true positives. I really can't use that email for anything as having to go to the spam folder every day defeats the purpose of an anti-spam filter.