Interesting. I found that my custom-___domain Fastmail address went from spam to the inbox in GMail the moment both SPF and DKIM were set up correctly.

I think the presence makes a difference, and SPF is often at least partially respected. But I think DKIM is so often incorrectly configured that most major mail carriers just decide they have to let it through anyway.

Yup, I've watched mail that I send be delivered directly to Gmail inboxes even if I spoof the From address to be from a Gmail account. (Admittedly, the rest of the headers show it correctly coming from the uwaterloo.ca mailservers, which are perhaps whitelisted, and the content of the email is never obviously spammy.)