Would Biff be suitable for building an API out of the gate?
Yes, via the :api-routes module key. You may have to fiddle with the middleware though (`com.example/middleware`)--I don't actually use :api-routes in any of my own apps, so there may be small/obvious improvements that need to be made. You'll also need to roll your own API authentication since Biff's auth module is only for humans visiting your site.
Cool.