I don’t believe that a generic estimate can apply to Openfire. Traffic depends greatly on the behavior of the users, the client implementations used, but also how things evolve over time. I’m not saying that your estimates are wrong - but even if they are right today, it is impossible to predict if they will still be correct tomorrow. Something as simple as a client making it easier to change a file, or use a higher resolution when storing an avatar for a user, or starting (or stopping) to cache avatars from users can generate wildly different results.
Your numbers are oddly specific. That suggests that you have been capturing and counting traffic on your server. That is probably the best approach for this: measure your environment, and try to extrapolate what this means if your user base grows. Still, expect things to be a lot different from what you expect.
It’s worth taking into account that most stanzas are text-based, which in size (in context of modern internet connectivity) is almost negligible for 500 users (we were doing this on mobile phones back in 2006 over 2G). If you start doing file transfer or video conferencing, that’s where things will take off.