CSS, especially when in external files, is often aggressively cached. You may need to flush all caches involved, both client side and server side, in order to see recent CSS changes.
style.css vs. Additional CSS benefits can be more complicated than initially thought. It depends partly on how style.css is served (CDN or domain’s server). Also on how caching is used. Taking an overly simplistic view, overall it’s the same amount of data either way, it’s not going to make a big difference IMO. FWIW, I prefer a child’s style.css over Additional CSS, not for performance but for organizational purposes.
Another consideration is CSS overriding default theme styling will get higher precedence as Additional CSS (which becomes inline styling) than it will in style.css. It generally doesn’t matter, but once in a while it’ll make a difference.