I think there are definitely more than 10-15 fics that go over 20,000 words by now. AO3's only got about twelve big bang-length fics tagged with Steve/Tony, but I don't think all the long fics that have been posted to the com have been uploaded there. Glasgowsmiles and therobotmuse's fics aren't up there, for one thing, and I'm pretty sure Letheward and Mouths of Madness are both at least 20,000 words.