You can have a look at the CometD benchmark results to have a glimpse of the CometD performance in a case that is worst than yours: in your case a user just chat with the owner, while in the benchmark a user chats with many others. The CometD benchmark is actually a normal chat application used to perform the benchmark.
With the WebSocket tranport, CometD easily achieves 50k messages/s for 100k clients with a 4 ms median latency.
My experience is that CometD works really well in real applications, and it's probably simpler than XMPP in your case.
Have a look at the CometD documentation and tutorials to get started.