though I'm not sure what all they'd share

If you haven't actually done anything to merit them investigating you (e.g. federal crimes, applying for a job with them requiring a background check) then not much. They certainly have access to a lot of data about you, but if they haven't slapped it in a report and filed it somewhere then it's not getting released under the FOIA. I only really bothered filing one for myself in the first place because I knew for sure there'd be something there.